In a nutshell, it's to do with strength. Just to contra the downvote, Brad Smith, author of beersmith has blogged about porter, and links to several recipes. I've looked over the recipes, and the darker ones could easily be taken as stout recipes, and the lighter ones, brown ale recipes.
I remember the guide telling at the Dublin Guinness brewery that they used to brew Porter. That was their main seller for a while, but in time started brewing "Stout Porter" - a stronger porter to remain competitive.
Wikipedia confirms this:
The name "stout" for a dark beer is believed to have come about
because a strong porter may be called "Extra Porter" or "Double
Porter" or "Stout Porter". The term "Stout Porter" would later be
shortened to just "Stout". For example, Guinness Extra Stout was
originally called "Extra Superior Porter" and was only given the name
Extra Stout in 1840.
So, a stout and a porter historically are the same style, but with the stout moniker implying that it's stronger. The same wikipedia page gives these names and gravities as a guide:
- Porter - 1.055
- Single Stout Porter - 1.066
- Double Stout Porter (Guinness) - 1.072
- Triple Stout Porter - 1.078
- Imperial Stout Porter - 1.095
So your 1.086 Imperial Stout has become a Single Stout (Porter).
Stout doesn't have to be jet black - a commercial example is Murphys', which is noticably lighter than Guinness, so don't worry over the lighter SRM.
When the brew is done, evaluate the hop balance, since that's quite a reduction in gravity - more boil utilization and less residual sweetness to balance. I'm sure the beer will be drinkable, especially given some time to mellow. I recently made a Stout with an unwanted biting hop bitterness, possibly from miscalculation/misweighing, but this has mellowed to a very drinkable beer after 3 months.
EDIT: although the same styles historically, there are are some recipe tweaks that are more typical of a porter: use of flavor and aroma hops, since stouts have typically bittering hops only, and the use of black (patent) malt for bitterness, where stouts tend to use roasted barley. But as with any rule, there are plenty of exceptions.