Hi Christine,
Both the 60+ Oyster card and the older person’s Freedom Pass are treated as off-peak travelcards so purchase of extension tickets is the same in both cases.
I’m a little confused about what you mean by changing at London Bridge in relation to Epsom, and specifically Ewell West. Are you arriving into London Bridge using Southeastern? If so, I’d recommend changing at Waterloo East instead and taking SWR direct to Ewell West from Waterloo. That way no extension is required.
Otherwise, you can purchase the ticket from Boundary Zone 6 to Epsom anywhere before you actually need to be using it. London Bridge would be fine. The return extension ticket is valid between either Ewell East or West and Epsom, so you could use it with your 60+ to get from East to West via Epsom. You’d need another return ticket to do the reverse journey later in the day because you use both outward and return on the one journey.
Hope that helps.