If you buy tickets near the end of your certificate's expiration for a refundable fare 330 days after the booking date, can you change the tickets again for a flight later than the original 330 days? How many times can you keep doing this? What if you have flown part of the ticket?

Here is what I'm trying to do:

I have a certificate expiring soon. The only thing that I anticipate being able to use the certificate for at this point is a one way from yvr-lax this summer. It saves a little to use the certificate, but certainly not a great value. LAX is home, though, so that seems to leave some options.

It looks like I can book a multi segment ticket with:
segment 1 yvr-lax (non refundable fare)
segment 2 lax-XXX (fully refundable fare)
segment 3 XXX to lax (fully refundable fare)
segment 4 lax-YYY (fully refundable fare)

We will fly segment 1 this summer. Sometime after that, when we are ready to make final plans, I would then change the dates, destination, fare class and possibly originating airports (say to leave and return from BUR if that ends up being more convenient) of segment 2-4 keeping the companion certificate in place. Will this work? How far in the future can those new flights be, 330 days from when I change the ticket, or from the original booking, or from the certificate expiration, or something else?

Any reason I shouldn't pick the cheapest fully refundable routes I can find for the future legs even if they are nothing we might want to fly?

Thanks!