Drive Letter On Boot Drive Changed When Plugged In To Another Pc

I wanted to move data from my old Windows 7 PC to my new Windows 11 PC. I removed the hard drive and plugged it in to an open sata connector on the W11 motherboard. Drive showed up fine on new computer.

When I moved the drive back to my old Windows 7 PC, it would not boot. It seems to be saying that it's attempting to boot from D. I'm wondering if when I connected it to the new machine, that machine changed the drive letter on it from C to D? Can it make that change on the actual drive? If so, I'm guessing that I need to use command prompt from the old machine and change it back to C. Is that a simple thing? I'm afraid of permanently "breaking" it.

I found these instructions but don't want to try them until someone who knows what they are doing can confirm that this will work and I won't mess things up further.

1. type "diskpart"
2. type "list volume"
3. type "select volume (NUMBER)" - number is whatever number is given to the disk I need to change
4. type "assign letter=c"

So my questions are:

1. Is my assumption as to how I "broke" it in the first place correct/plausible?
2. Is the issue easily repairable and will the command prompt commands I listed above work to solve the problem?

Thank you!