Hello aamirawan91

As a general rule, when you replace a CPU with its compatible successor, the STEP 7 project does not need to be changed. 

Also, note that successor that you have proposed for the second CPU is 6ES7515-2AM02-0AB0, which is also phased out. The successor of this one is  6ES7515-2AN03-0AB0 but, as there are two jumps between successor products, you proably won't  be able to perform a plug and play replacement.

I think you'll find useful this FAQ: How do you find the right successor product if the existing product has already been discontinued? 

Good luck!