hi Dee,
I agree with Linda. Zero communication. Just set the line on when your start date will be and when theirs ends.
When we took over a solo group, we took all the contracts he had on hand and listed down all the insurances in his area and started calling. We started from scratch. We organized all his contracts with the payers as well as researched if he were or had whatever we needed, enrolled in EFT/ERA etc or if he had a provider number already with BCBS etc. We fixed his CAQH as in everything. We also cleaned up his aging and credentialed him with the rest of the insurances.
Its hard work to do a take over but its very fullfilling once you know you made a difference when you organized your providers credentials and the provider is starting to see money coming in his bank account.
Michelle and Alice have wonderful books that have helped me a lot.
Hope this helps. Let me know the outcome of your hard work