If you want to temporarily bridge the two VLANs, use a pair of switches with STP disabled on them.
Bridging the two VLANs could be a valid way to migrate a network into another. I know I've done that before, but only in special circumstances.
I think in your situation, I would prefer to enable VLAN 172 on a switch-by-switch basis: pick a switch, enable VLAN 172, and change all the switchport VLAN configs to replace VLAN 1 with VLAN 172, assuming inter-switch links currently have VLAN 1 untagged. This way, as you go along, the VLANs are bridged on every switchport that connects any migrated VLAN172 switch with any unmigrated VLAN1 switch.
As far as avoiding network downtime goes - you should be doing this out of hours anyway.
If your servers are in the same subnet as other hosts, then perhaps moving to multiple subnets/VLANs is a better idea anyway - it is usually best to avoid spanning any VLAN across multiple access switches if it can be avoided, and you should segregate clients from servers.