If the monorail is opening late, buses will be running from TTC. If you're able, walk to the TTC (it will be faster than the Resort Monorail or replacement buses), and ask at the Bus Info booth where to get the bus to Epcot.
But I would not rule out the possibility that buses will be running directly from Poly, as transportation info provided by resort cast is often inaccurate. Unless you're room is closer to the TTC anyway, stop by the bus stop and see if a "greeter" is stationed there; they will usually be a college kid wearing white shirt and black shorts or pants. Ask the greeter if buses are running to Epcot; they will know. If they're not, you will have added only a minute or 2 on the walk to TTC.