Changing hot water supply and keeping cold water supply
Your cold water line connection if up to code is not an issue as to where it comes from. What I mean by 'if up to code' is that it was originally installed up to code.  

The short answer is you need not do any other work regarding that cold line unless you really are pitching an itch to do it.

The only thing that I see (really a non-issue) will be that for any future issue a person will need to know the hot water comes from the additions tankless hot water heater for your sink and your shower. Again not really an issue - just a be aware that this is the case.