Good plan. But if it's been down to zero for awhile, the receiver/drier should probably be changed.
A good plan might be to screw in the new switch in the old drier, vacuum the system (and wait a pretty good while, 2 hours or more to be sure).
Then if all looks good, pop the new drier on with the new switch, and do the vacuum again. After that charge her up, and add one can that has UV dye, just for furture leak detection ease.

Remember to research on the qty of 134 required. I'm pretty sure your system needs 46 oz (but double check me on that, the wife has the car today) of R12 so it should require maybe 32 oz or so of 134??

DG