Major game flaw...

Technically endurance covers SP, RP and SP/RP. So on a technicality they did tell you this could change. Though I 100% agree it should not have changed so quickly that your waivers were destroyed.

Well, in this case the difference wouldn't be in terms of endurance but designation.

it all stems from the 4 'pure reliever' rule-- or course which there is no such thing in mlb. It would be fine in the game as well if it weren't for the fact that they were switching designations on guys.

unfortunately, sometimes (myself included), some folks don't learn their lessons very well.

For example- what would be the best way to get people playing this version of the game?

Ive already given the answer earlier.

SOM stated that hitters positions won't change.

Why not? If a guy starts playing a new position, that position should be added to his card after he plays a few games there. (P.S. That's what fantasy baseball does, and I think it works out well.)

Fantasy baseball adds positions but does not subtract. You have to grandfather position eligibility or players might not be able to field drafted teams a month into the season. SOM should do the same.

SOM stated that a player will not "lose' a position but a new position may be added. This includes pitchers.
I wish they made that decision retroactive as I drafted a player with a SP/RP (carlos martinez) and after I drafted him his position was changed just to SP.