RDS SQL Server - scaling up down instance size

Is there a work around to scaling an RDS MS SQL Server instance type/size down (with no downtime)

In every situation there will be some downtime, Its how that downtime and technology that is used that matters more. For your question, the best approach would be if the customer is using Standard or Enterprise Edition, is to have the database in Multi-AZ mode and scale the instance to the desired class, this will have approximately 60-120 secs of downtime. For Single AZ + Web and Express Editions, it would also be preferred to scale the instance class using the modify operation if the desired outcome is to simply scale the compute (For SAZ scale compute will take approx 15 mins to complete.)

https://aws.amazon.com/blogs/database/scaling-your-amazon-rds-instance-vertically-and-horizontally/

Building out a new RDS instance from a snapshot as that has down time

Has the side effect of lazy loading, and the customer will encounter higher latency for the duration the storage is downloading blocks from S3, Scale compute operation will work better in this case.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-restoring-volume.html

There are other Techniques eg using DMS where you can replicate the Database to the desired instance class and then cutover, but this would be an overkill for the outcome you are trying to achieve, as there would still need to be a small window of downtime for cutover from the existing instance.