You can't, once you fill all the slots you have to keep the ones you have.
to get a pokewatch in jublife city
In Pokemon pearl, diamond, and platnium, when your done at the trainers school in Jublife city
You can get one in Solaceon Ruins with the dowsing machine app on the poketech.
If you are talking about in Jublife city, you go talk to the guy who you first talked to about the Poketech and you get the Poketech. Then you go up then to the right and out the ramp where the guy wouldn't let you go before on to the first town.
You get it from a women on 1FL of the Pokemon Centre in Eterna City.
find clowns in jublife city then talk to poketech guy
You can get it from the PokeTech App place in Jubilife City.
go to jubilife city and talk to the poketech directer (he is on the first floor once you walk into the poketech building). you must have at least 5 or 7 badges first
If you have the marking map it will track mesprit, just don't fly.
Well acording to alot of people, you can't get rid of a missingo on diamond.
you cant get rid of key items.
the action replay code to have all the poketech apps is 94000130 fcff0000 b21c4d28 0000000 b0000008 0000000 000013b8 01011903 c0000000 00000017 200013bc 00000001 to activate press l and r