EC2 instance CPU burst credit behavior

Hello.

When an m6g is modified in place into a t4g, what is the burst credit balance when the t4g starts?

I looked for documentation, but couldn't find it, so I started EC2 and tried it.
I started one EC2 of m6g.medium.
I changed the instance type of that EC2 to t4g.medium.
I think that when you change the instance type, the count starts from 0.

The image below shows metrics recorded about 10 minutes after changing the instance type.
The first number recorded after changing the instance type is approximately 1, so it is probably counted from 0.

As stated in the document below, t4g.medium can accumulate up to 144 CPU credits.
Also, since you can get up to 24 CPU credits in 1 hour, it is thought that you are getting about 2 CPU credits every 5 minutes.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-credits-baseline-concepts.html#earning-CPU-credits

I think a possible reason for the count starting from 0 is that there are no launch credits.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-credits-baseline-concepts.html#CPU-credit-accrual-limit

T4g, T3a and T3 instances do not earn launch credits. These instances launch as unlimited by default, and therefore can burst immediately upon start without any launch credits. T3 instances launched on a Dedicated Host launch as standard by default; unlimited mode is not supported for T3 instances on a Dedicated Host.

also, if the instance starts at t4g, accumulates say 4 credits, scales to m6g and then back to t4g a few days later... does the t4g still have its 4 credits?

This would take some time to try, so I tried changing the instance type without delay.
The instance type was changed in the order of t4g.medium → m6g.medium → t4g.medium.

The image below shows the metrics before changing from t4g.medium to m6g.medium.
There are approximately 10 CPU credits.

Let's look at the numbers after changing to m6g.medium, waiting 10 minutes, and changing back to t4g.medium.
The image below is an image of the metrics when changing back from m6g.medium to t4g.medium.
After changing the instance type, the first number of CPU credits recorded was approximately 12.
As a result, no CPU credits were lost.

Perhaps, but CPU credits will be retained for 7 days.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-credits-baseline-concepts.html#accrued-CPU-credits-life-span

For T4g, T3a and T3, the CPU credit balance persists for seven days after an instance stops and the credits are lost thereafter. If you start the instance within seven days, no credits are lost.

Please note that the above results may vary depending on the application and workload you are running.
In the tests I conducted, I checked without any applications running on EC2.