Hi Mayuresh Halarnkar and thank you for posting,
It sounds like this problem is only happening for existing forms when you are in edit mode.
My guess is that when you migrated the values for your lookup, that the IDs changed. So, when the old XML form opens and tries to get the value, it's no longer there. The lookup values would have been to another list or library, right?
If it's read-only, it doesn't try to re-query the list. This suggests that the reason it is failing is that the lookup value list name changed. Why do I say that? Well, if you're using the list IDs they will be natural numbers, so even if the rows don't migrate in the correct order, the IDs should still be there, so it should find them. That is, unless the data connection is still broken.
Please check your data connection and make sure the lookup is correct.
Good luck,