This is the advice a cast member just gave me:
“Typically while Disney’s Blizzard Beach Water Park is under refurbishment, you’d board a bus from either the Theme Parks or Disney Springs® to Disney’s Coronado Springs Resort, then hop on their Epcot® bus, letting the bus driver know you want to stop at Winter Summerland. They’ll drop you off there before they head off to Epcot®.”
This doesn’t make much sense to me given the proximity of Coronado, Epcot and Blizzard Beach, but that’s what they recommend.
6/10/17 Update: Asked a different cast member the same question (because this advice seemed so ridiculous). Now I’ve been told that the Blizzard beach bus will still run to provide service to the Winter Summerland. This doesn’t really sound right to me either because Disney would have a bunch of empty busses burning fuel. I keep waiting to be told to talk to the concierge to secure transportation (like for early ADRs). I’ll keep you all posted…