Your questions are not really related to Cisco, aside from the last one. That link only mentions you porting your DNs, regardless of what call control you use.
1 Ask MS, if they will become your service provider, is there a way to back out? to port them to other service provider? etc. All that, you should be asking MS.
2 No idea what your plan is, so, that's up to you, if you remove the PSTN connectivity, and want to keep using CUCM for internal purposes, you can do it.
3 Again, questions you should be asking ATT, not Cisco, if ATT is your current telco, this is a discussion you need to have with them. From a CUCM config, that would depend on what you will do, if you will remove CUCM, you could simply delete the DN and remove/reassign the phone. If you will remain using it for internal purposes, you can give it a new DN.
Doesn't seem like you're familiar with voice, I'd strongly suggest you hire reputable consultants to assist with this and determine exactly what it is you want to do, and then how to proceed accordingly
if this helps, please rate