Any reason why you're using a PRI handoff? IP Flex SIP integration is pretty straightforward and eliminates the need for a T1 card.
We do these kind of flips all the time... Customer has a PRI and moves to voice SIP service but doesn't want to invest in the licenses for SIP trunks, just reuse the PRI card. It's generally as problem free as SIP trunks are. Most companies provide Adtran TA series devices, and honestly as long as the power is on a good UPS, these will generally have better uptimes and be more problem free than the VoIP carrier's switches.
And to Willtech, these are usually fairly simple as PRI's are largely standardized now. Most likely you just need to move the cable, or make a T-1 crossover cable (remember this is different than an Ethernet x-over cable) and plug it in. If it doesn't work, verify you are set to NI-2/Bellcore and in general everything works.
You usually don't have to mess with the digits in a 5000, it will match based on the last digits of the DNIS digits, so if your CRA is built on 4-digits and the new carrier is sending you 10, the 5000 will essentially ignore the first 6-digits and match accordingly. These kind of change outs are
usually pain free and simple.