Unless it requires a complete rewrite (á la XF 1.5 to XF 2.0) then yes. Once XenForo 2.2 has been released, development will shift towards 2.2 and the 2.1 version will no longer receive updates.
If 2.x to the eventual 3.0 does require a complete rewrite, whether an upgrade will cost money will depend on the amount of work involved.