Issue while changing instance type to t3a.large from c6i.xlarge

Based on the issue you're experiencing, there are several possible reasons why your instance won't start after changing from c6i.xlarge back to t3a.large:

Possible Causes:

1. Architecture Compatibility: The c6i.xlarge and t3a.large instance types have different processor architectures. The c6i instances use Intel processors, while t3a instances use AMD processors. When you performed the Windows upgrade on the c6i.xlarge instance, the system may have installed or configured drivers and components specific to the Intel architecture that are not compatible with the AMD architecture of the t3a instance.

2. Enhanced Networking Drivers: The c6i instance type supports enhanced networking and requires specific drivers (ENA drivers). If the Windows 2025 upgrade modified or updated these drivers in a way that's optimized for the c6i instance type, there might be compatibility issues when switching to t3a.large.

3. NVMe Support: Both instance types are Nitro-based and expose EBS volumes as NVMe block devices, but the Windows 2025 upgrade process on the c6i instance may have configured NVMe settings that don't translate well to the t3a instance type.

Recommended Solutions:

1. Verify Architecture Compatibility: Ensure that the t3a.large instance type is compatible with your Windows 2025 installation. The in-place upgrade performed on the c6i.xlarge may have installed architecture-specific components.

2. Check Driver Compatibility: While running on c6i.xlarge, verify that all necessary drivers for t3a.large (particularly network and storage drivers) are properly installed and compatible with Windows 2025.

3. Consider Using an Elastic IP: If you need to retain your public IPv4 address during instance type changes, use an Elastic IP address to avoid connectivity issues.

4. Alternative Approach: Consider creating a new AMI from your current c6i.xlarge instance and testing it with a new t3a.large instance before making the permanent switch, or consider staying with an Intel-based instance type that matches the architecture of your upgraded system.
   Sources
   Troubleshoot changing the instance type - Amazon Elastic Compute Cloud
   Compatibility for changing the instance type - Amazon Elastic Compute Cloud
   Troubleshoot Amazon EC2 Auto Scaling: AMI issues - Amazon EC2 Auto Scaling