The same way you restructure any contract? You pay him more now as bonus so you can spread it over years, and less base salary later. Yes it was guaranteed either way, but he benefits by getting it now.
I could be wrong (I don't think so because I've seen talk of restructuring Watson) but I don't see why guarantees would change that.
Having looked at a few contracts, nearly all high-dollar NFL contracts of 5 or more years are structured so that year 1 is nearly all signing-bonus, year 2 is an option bonus that's guaranteed at signing, and the remaining years are base salary. The NFL also allows you to do a "simple" administrative restructure without requiring player consent, converting any year's base salary to an option bonus for the remaining years of the deal. In order to gain more flexibility with this you can extend the number of years on the contract by including void years or non-guaranteed years that are wink-and-nod cut years unless the market goes absolutely nuts.
Typically the idea of a re-structure is to convert non-guaranteed money (base salary) into a bonus (guaranteed) in order to kick the can down the road cap-wise. In this instance, it'd be converting guaranteed money into guaranteed money in another form. The only benefit to the player is that he'd get that bonus money up front rather than having to wait for it, but he isn't gaining any dollars from such a move.
Base salary and roster bonuses can be made guaranteed at signing.