Connect an acquired company that uses the same 10.1.0.0/16

Your company (HQ) has bought Acme, 120 people in one office. Both companies use 10.1.0.0/16 internally. Legal close is Monday, and from day one Acme staff must use HQ's ERP at 10.1.20.15, and HQ IT must manage Acme's servers. An IPsec tunnel between HQ's interconnect router R1 and Acme's router ACQ-R1 is already up, with eBGP across it (HQ AS 65010, Acme AS 65020). Plan the connection now and the end state later.