
Before selling your MPC X go to 'PREFERENCES > ACTIVATIONS' and deactivate your plugins on that machine, probably worthwhile doing the same in the MPC Software.
You should then register the MPC One in your account, so in the first instance try entering the MPC One hardware serial in your account and see what happens. If it's used you might get an error ('already registered') in which case you'll have to contact Akai to have it manually added to your account.
Once rthe new MPC is registered, log into your inMusic account on the MPC One and activate your plugins. Do the same in the MPC Software - you might have to first re-activate the MPC Software as Akai might have assigned you a different iLok serial (as the old serial is linked to your previous MPC X), but I'm not sure if they bother with this.
It's not normally a problem, other than potentially having to wait days for Akai to get back to you, so I would suggest you provide as much info as possible in the first instance to avoid having to go back and forth with support.