I did not know you could upgrade the free bring a friend passes, good info. I was just going thru ours and threw a bunch away. We have not used any of our free passes so far this year. I think the next ones are good in July
So I'm going to hedge my statements just a bit here. I'm fairly sure that the bring a friend free tickets can be upgraded, but I'm not positive. There is a chance that they might not be.
I know that they will upgrade the free tickets they give employees at a discount. And other tickets that the park gives out free, such as for promotions, the teacher day, and things like that, they will also upgrade at that discounted rate, along with all tickets that they've actually sold- including discounted or group tickets.
However, there isn't any statement in the annual pass holder information on the tickets or the flyers stating that the tickets can be upgraded to a season pass. So there is a chance that those tickets are not eligible for the upgrade. I've never upgraded one of those, but I have upgraded or been with people that have upgraded all of the others. It seems like it would be strange to make the "bring a friend" tickets the one exception to that, but it is possible.
TLDR- be sure to double check and confirm with the park that the "bring a friend" tickets can be upgraded to season passes before you make plans to actually do it. :-)