need alternative for "EP2C15AF256I8N"

Well, the Quartus 13.0sp1 (supports CycloneII device family) has a number of smaller/larger cell count devices that are footprint compatible.

Depends on if you want to stay in the same (now EOL/obsolete) device family.

I doubt you will find any newer devices CycloneIV/V/10 or MAX5/10 that are drop-in footprint compatible.

That will take a bunch of work on your part reading data sheets and comparing device pinouts.

Unfortunately the Quartus migration tool does not help very much when going between device families.