Hi David.
SRM 5.0.2 was released after 5.1, so there was no code built into 5.1 that is aware of the upgrade path from 5.0.2.
Good news is that there should be a new 5.1.1 release within the next 24 hours that will have that capability... so your upgrade path is now straight from 5.0.2 to 5.1.1.
You'll probably spend a lot more time on the vCenter upgrade with all the new bits there than with the SRM upgrade - it's pretty straightforward.
Hope this caught you in time before you rebuilt.