I just came to the realisation, that if your TP1500 is part of a TIA project that also includes an S7-1500 PLC, then it is maybe not a good idea to upgrade to V15.1 if your contractor only has V13.
The reason is that when you migrate to V15.1, TIA will demand to compile all program blocks which in turn demands a complete download to the PLC, which in turn means that the contractor with V13 no longer can go online.
Even if you do not change the firmware version of the PLC, TIA still demand a compile and a renewed download.
This is the worst issue with TIA today IMO. The TIA version must be the same, for all persons working with the project. It makes maintaining existing projects with older versions a pain, and forces you to maintain multiple VMs with previous TIA versions.
If you only have a TP1500 in the TIA project, then OK to upgrade to V15.1.
Or if the PLC is actually a "proxy" PLC, then it is possibly also OK to upgrade to V15.1