When you go on the Southern site to book tickets (to see what trains are actually running from Polegate to Falmer) they all show change at Lewes for 29 Feb. Go figure.
https://www.southernrailway.com/travel-information/plan-your-journey/planned-engineering-work?date=2020-02-29#engineering-departure-board
Buses from Eastbourne to Polegate.
Plus no apparent direct trains from Polegate to Falmer.