How easily can asics be ported over to other sha algorithms? Let's say I want to rewrite the software behind the hardware of current asics. Would it still mine, for example, sha1 algorithms orders of magnitude faster than a normal cpu would?
Application Specific Integrated Circuits are hard set to only do one thing, and in the case of bitcoin ASICs, that's SHA256 hashes. You cannot re-write the software, you would need a new circuit built that only does SHA1 or whichever hash you're looking for. 

https://en.wikipedia.org/wiki/Application-specific_integrated_circuit