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i'm not sure whether you are still looking for answers for this.
7 to 9.1 is one step upgrade path.
But of course, if you want to make use of WinDU, you may need to do an extra step, if you are source is earlier MOR of 7M050.
7MOR to 9.1MOR
7MOR to 7M090 (windu supports M70 onwards, but M090 is latest one) and then upgrade to 9.1MOR
as you know, Upgrade is always dependent on the size & quality of data you have.
If you have CAD Data, you may end up in FT related issues while upgrading,
one more common issue with upgrademanager till 9.1M050 is that there are some migrators which run very slow, solution for them is skip them in UM and execute the OOTB utility for the same migrators as post upgrade step
and do you have any specific modules like MDT or Partslink??
DISCLAIMER: Author doesn't responsible for any unforeseen things happened due to implementing the suggested process