You can use the existing location as a junction box, then route new cable to the new location. Be advised, the new panel will have to comply with all current national and local codes as to location and clearances (not in a closet!).

Q: Single Family Home, or multi-family / mixed use residence/multiple "service" (apartment present)? USA? What Electrical Codes apply in your jurisdiction? "Service" elsewhere on your property, being FEED to this garage location?

Q: Fire pump connected to this service?

Q: What brings you to consider moving this panel? Do you have an accessibility/clearnace violation concern regarding this closet itself and/or its stowage/storage uses conflicting with same?

Remember that service conductors are connected to the line side of the service disconnecting means and feeder conductors are located on the load side. Knowing this distinction isn't enough, a sound knowledge of a multitude of other installation parameters as outlined by the Code, is imperative to provide for a sucessful, safe, and compliant job. Assuming that some version (or equivelent) of the NEC is in effect for your jurisdiction, and assuming you're in the United States, I'll refer to the NEC. citations are from the 2005 NEC, but the NEC has remained fairly consistant since 1999 regards to these determinations (although the code has been re-enumerated and re-arranged significantly since the 1999 edition).

You may not install service conductors inside a building [230.70(A)(1)]. In addition, you may not install service conductors in the same raceway or cable with feeder or branch-circuit conductors (230.7). This arrangement could bypass overcurrent protection for the feeder conductors if a fault occured between the service and feeder conductors.

If this "Main Electric Panel" is on the exterior side of the structure, is the first point of disconnect for the "load" side of your electricity from the power company, and is the location where your "line" is comprised of non-grounded conductors - then NO.

If your "neutral" is bonded to ground within this "main electric panel" (thereby creating the "grounded conductor" for your "line side" of premisis supply)and contains your primary "overcurrent protection" for the entirety of same; then basically the short answer is: No, you MAY NOT just remove the panel componants, and "splice" wire from this present location, to a interior location and then re-create your "main Electric Panel" inside/elsewhere.

At the point of your service entrance to the structure you are going to need a bond to ground and an overcurrent protection device and a means to disconnect. (like a CB main). Then you can run a FEEDER circuit with both your phase "hots" conductors, your "netural" (grounded conductor) and your "ground" (grounding conductor) to an interior Feeder panel, with another main/disconnectiong means/overcurrent protection, which then provides for overcurrent protection and deliniation of your various branch circuits).

Nothing "unprotected" on the "load side" of your service connection is allowed to be "junctioned" or wired within the "premisiss" side of the structure. A simplified "main" (with disconnect, overcurrent protection and bonding jumper, main) could perhaps be created upon the outside of the building, or the entrance point relocated elsewhere on the exterior wall that is beyond this closet, or the closet re-configured so that this panel/closet storage situation is no longer an accessibility and clearance violation condition.

These are the terms that you need to define depending on what version of whichever code (however modified) for your jurisdiction applies (definitions for example from Article 100 of the NEC, version that applies to your jurisdiction, should that be applicable to your locale).

Service

Service Point (further determined by both the supplying utility and local codes), the Service-Drop conductors and Service-Lateral Conductors may or may not be a part of the Service.

Service Conductors

Service-Drop

Service-Entrance Conductors (underground system or overhead system)

Service Equipment

Service Equipment Enclosure

Service-Lateral Conductors

Premises Wiring

Feeder-Circuit Conductors

Accessible (equipment & wiring methods)

Accessible, Readily

Dead Front

Disconnecting Means

Device

Duty, Continuous

Dwelling (Unit; One-Family; Two-Family; Multifamily)

Enclosure

Equipment

Exposed

Feeder

Panelboard

Premises Wiring (System)

Qualified Person

Authority Having Jurisdiction

Utilization Equipment

Chapter 2 Wiring and Protection

Article 200 Use and Identification of Grounded conductors

Article 210 Branch Circuits

Article 215 Feeders

Article 220, Branch-Circuit, Feeder and Service Calculations

Article 225 Outside Branch Circuits and Feeders

Article 230 Services

Article 240 Overcurrent Protection

Article 250 Grounding and Bonding (esp. section VI Equipment Grounding and Equipment Grounding Conductors)

Chapter 3 Wiring Methods and Materials

Article 408 Switchboards and Panelboards