...but I thought TIA Portal was current, and Step 7 was Legacy. ???
I think from a support standpoint, "legacy" means "we haven't sold it in 10 years". S5 is legacy, but Siemens is still selling S7-300s and Simatic Manager/Step 7 v5.6. Maybe the ORIGINAL S7-300 CPUs are legacy, and I'm sure old versions of Step 7 can be tricky, but, as you found out, they're pretty forgiving about that.