Normally it is illegal to build an electrical outlet inside a cabinet — for example — in your kitchen. The reason is that if you were to plug in an appliance like a toaster and close the door, you could have a fire on your hands.
At Easy Installations we have come up with a method to do this legally. We set up the outlet so that it is only active when the cabinet door is open. This way you can use your toaster or coffee maker in your appliance cabinet if you wish — no need to move them to the counter. Pretty cool idea eh?
All junction boxes in use, whether with receptacles, splices, switches or other devices, must be accessible.
This is probably the generally "best" answer. But it may involve significant work, depending on the type of wall. However, not only can receptacles not be covered, junction boxes can't be covered, even if they are only in use for a splice.
If a cable comes down from above, moving the receptacle up is easy.
If a cable comes up from below, you have a problem because you need to extend the existing cable. Normally you do that by splicing in an additional cable in the junction box. But that doesn't help here because now you have a junction box that still can't be covered. There are splices that are approved for in-wall use (i.e., not accessible) which may be an option.
Depending on the type of cabinet, or specifically the type of cabinet back wall, you can:
With typical walls, you could use the receptacles below and it should be very easy to install receptacles above the counter and drop cable down to the lower receptacles. Pigtail each set of 3 wires (in, out, new) in the lower boxes, even if originally they had the normal in on one screw, out on the other screw.