What Cassius said. It is probably of lesser dollar value. And it's difficult to refinish over tung oil, just due to the nature of tung oil being a penetrating, hardening oil.
I understand why you did it. I actually specified tung oil on my custom fanned fret bass that has an HB-1 pickup in it. I know the neck feels great and is a pleasure to play.
Now, think about it: this is your bass. You're the player. You got the bass when it was relatively new, and made it fit you and your playing, knowing at the time more were being made. George Gruhn talks at length about "players' instruments," and how the tools of the trade have to fit the player to be really useful, not just "collectible" by some abstract standard.http://www.premierguitar.com/Magazine/I ... cians.aspx
Bottom line: you did what you did. Unlike changing a pickup, knob, or pickguard color, it is irreversible. So what! What you did made your bass playable for you for all these past years of enjoyment, and all the ongoing years of enjoyment to come. The bass is still good bass. I would focus more on the enjoyment of the bass and the fellowship of the people I get to jam with playing it, and less about any abstract dollar value.