I have seen some posts recently about LC priority dates. Since mine being a LC substitution case, I spoke to a reputed lawyer (Immigration-Law.com) and this is what he said about LC substitution priority dates.
If LC is applied and approved for Alien A by a company but I140 is never applied using that LC and if Alien B is substitued using that LC, then Alien B is eligible to get the original LC priority date.
On the other hand, if LC is applied and approved for Alien A and I140 is filed by the company for Alien A, I140 is approved, then it is revoked and then the LC is used for Alien B, then Alien B cannot use the original LC priority date and I140 RD will be the priority date.
The third possibility is if LC is applied and approved for Alien A and I140 is filed by the company for Alien A, I140 is revoked before it is approved and if Alien B is substitued using that LC, then Alien B is still eligible to get the original LC priority date.
The word eligible is important and tricky because Alien B can still have a priority date other than and even before the substituted LC's priority date, if he has an LC and 140 approved through another company.
This is my understanding from the personal telephone conversation with this reputed lawyer, but there are other gurus and experts in this forum who can explain better.