size modifications in standard RI and EC2 Instance Savings plan.

In general, Savings Plans are much more flexible and are better for most users. In answer your specific questions:

Which of them allows to change OS in future.

Savings Plans and Convertible RIs allow you to change OS, Standard RIs do not.

Does either of them allows to change instance from c6g to c7g or from c6g to c6a?

Savings Plans give you flexibility on any compute type. Convertible RIs can be exchanged across instance families. Standard RIs can change instance size but not family.

If I buy a c6g.xlarge standard RI for one year today, and want to resize it to 2xlarge after say 6 months, (as per aws docs, I can do this by buying another c6g.xlarge, say I buy it for 3 years) what will be the validity of the newly generated c6g.2xlarge RI, 6 months or another three years from the day of latter purchase?

With an RI, if you buy a single c6g.xlarge, then after 6 months you want a 2xlarge, you can buy a second c6g.xlarge reservation and the two reservations will be automatically applied to your c6g.2xlarge instance. When the first reservation expires, you will then have only one RI for a c6g.xlarge and it will not be sufficient to cover your c6g.2xlarge instance.

What will happen in case of EC2 saving plan for the same scenario as Que. 3?

It's a bit different with a Savings Plan because you don't buy for an instance type, you buy a commitment for a certain dollar per hour spend. So if you commit, say, $15/hour for a 3 year term, then after 6 months you decide you need $30/hr, you can add another Savings Plan for an additional $15/hour with a 3 year term. Now you have $30/hr for the next 2.5 years, and $15/hr for the 6 months at the end of that term. During this period you can use any instance type that you want and it will benefit from the discount, up to $30/hr.

Be sure to review the RI docs, especially the Modify Reserved Instances page because it lists some important constraints to be aware of.

1. Which of them allows to change OS in future.

EC2 Instance Savings Plans apply to running instances within a region, and within specific instance family & generation (e.g. M5 instances in Ireland region, regardless of the size). So, EC2 Instance SP allows you to change OS of your running instance, as long as you stay within the same instance family & generation within that region. With Standard RIs, that's not possible - when you reserve Standard RIs, you have to stick with the OS that you chose when purchasing it.

However, if you were interested in Convertible RIs - those allow you to switch OS of your running instance. In that case you would use RI exchange process to change the OS of the RI to match attributes of your running instance, too. But also - if you had Compute Savings Plans - note that they just cover whatever :) you can change your running instances whichever way you like - Compute SP discount will still apply without any effort on your side.

2. Does either of them allows to change instance from c6g to c7g or from c6g to c6a?

No. Neither Standard RIs, nor EC2 Instance Savings Plans allow that. You need to stick to the original instance family & generation, within a region where you make a commitment.

However, again, if you were interested in Convertible RIs, you could change your running EC2 instances from c6g to c7g or from c6g to c6a, and then you could convert your RI into another instance family & generation using exchange feature. Similarly, if you had Compute SP - it doesn't matter how you'd change your running EC2 instances, as the discount would apply automatically anyway to the new instance type.

3. If I buy a c6g.xlarge standard RI for one year today, and want to resize it to 2xlarge after say 6 months, (as per aws docs, I can do this by buying another c6g.xlarge, say I buy it for 3 years) what will be the validity of the newly generated c6g.2xlarge RI, 6 months or another three years from the day of latter purchase?

The answer depends on the OS of the underlying instance. For Standard RIs, only Linux/UNIX RIs, with Regional scope, and Shared tenancy are size-flexible. It means that you can buy an extra .xlarge RI, and then have 2 of those .xlarge RIs apply to one running .2xlarge instance, only in case of size-flexible RIs. Then, let's say you have 1-year RI applying to your running instance, and after 6 months you decide to increase the size of your running instance. You could then buy another 1-year standard RI of the same size, and have both of them cover to your running instance (each would cover 50% of the instance). The new RI would be purchased for the full term, UNLESS you find the RI you need with shorter term on the RI Marketplace where you can only buy Standard RIs.

However, if you're for example, running Windows/RHEL/SUSE - those are not size-flexible. And if you had .xlarge RI, covering one .xlarge Windows instance... and you wanted to upgrade your instance to .2xlarge, then your RI cannot apply to that instance anymore, because the size has to match exactly in this case. With Convertible RIs, you could just use exchange feature to change your RI, to cover your new instance size in this case, and the end date of your RI would still be the same (i.e. you wouldn't have to purchase an extra RI for that, you'd just convert the existing one).

What will happen in case of EC2 saving plan for the same scenario as Que. 3?

EC2 Savings Plans would simply apply to your new instance size (as long as it's in the same family & generation... like... M5 for example). Same with Compute Savings Plans - they just apply to your EC2 running instances regardless.