According to paulheu in this thread
, post #10, "An announcement was made that the Insider program members will be able to upgrade to the final release at no cost." I only recently signed up as an insider, so I don't know exactly what announcement he's talking about, but if I run across it, I will post here again. Also, that technically doesn't say anything about upgrading straight from TP to final release, only cost, but it sounds to me like it should be easy enough. Maybe ask him if he can give you the source of his claim.
My main recommendation is to create a recovery drive to be able to downgrade back to W8 in case you do
need to in order to upgrade. And really, you should be doing that before installing the TP anyways.