We currently are running Border Manager 3.7 with a site to site VPNs from
our main office (master) to two remote offices (slaves).

I want to replace the BM 3.7 server by installing new hardware and BM 3.8
(3.9 ?). At remote site #1, I want to do the same thing (new server and
BM upgrade). For a while, I would have both the 3.7 and the new 3.8
servers running and available.

We are not ready to upgrade site #2 at this time.

Once I get the new hardware running at the main office and site #1 with BM
3.8 (3.9 ?) running, do I need to keep the old BM3.7 server online for
site #2, or will the BM3.7 slave at site #2 connect and work with the BM
3.8 master at the main office ?