[LetBinding]

Again you are technically right. But in practice this is of no help.

The AC21 allows you to change employer when you have filed the I-485. That is step 3 of the 4-step green card process. One can't file the I-485 until the "priority date" is current. That is not an issue if you are from any country other than India, China, or Philippines. Else you are looking at a delay of 4 to 11 years. So as I said, not practical for the poster who is from India.

Technically it is also possible to accept "normal career progression" changes in employment, i.e., promotions and raises, as you correctly pointed out. However in practice, what constitutes as "normal" is subjective, completely at the discretion of a visa officer. For a long time, this was very easy and a formality. In the last 1-2 years, the Visa Officers have become anal about this. Cases are being audited for something simple like Data Scientist using Python to Data Scientist using R (actual anecdote in my company).

Also visa officers are going back to approved cases from several years ago and retroactively denying them on some technicality like the above. So even if you know cases that were easily approved in the past, they can be suddenly denied and the employee can retroactively become "undocumented" for the past several years (again actual anecdote).