How far off the floor should my circuit breaker box be? I'm changing from a 100 to a 200 amp. The lead in wires are underground and come into the bottom of box making them too short to connect.
I don't think there's a minimum height, but there's a maximum height for breaker handles.
240.24 Location in or on Premises.
(A) Accessibility. Overcurrent devices shall be readily accessible and shall be installed so that the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, is not more than 2.0 m (6 ft 7 in.) above the floor or working platform...
So for a main breaker panel, where the main breaker is at the top. The center of the main breaker handle, cannot be higher than 6' 7".
Check with the local building department, as they may have a minimum height.
Practically speaking, you'll want it at a comfortable height to work in. You're not going to want to sit, or kneel on the floor while working in the panel.
You say you are replacing a 100A panel with a 200A, but the wires won't reach??? It is almost a certainty that you will need to replace the wires. It is extremely unlikely that they originally ran wire large enough for a 200A service, yet only installed a 100A panel.
Also, with pretty much all new main-breaker panels, the breaker can be mounted top or bottom, meaning there is no such thing as an "upside down" panel. Having the main breaker on the bottom will make for a cleaner and simpler installation where the feeders come in the bottom.
You should check if your new panel (or return it and buy a new panel that) can have the main at the bottom. My 200 amp panel (Sq D QO - evidently "convertible") is labeled/listed for both directions. My habit-following electrician (I do many things, but hired a pro for the main service connection) was about to complicate life considerably by trying to put them at the top when I pointed out that not only could they be installed main-on-bottom, but the customer wanted them that way ;-) and it simplified the installation considerably, since power was entering at the bottom. On that particular panel there's no potential confusion about "where's the main", since it's 4 times the size of a regular breaker and extends across both rows of regular breakers. You might look on top first, but you'd see it on the bottom right quick.
You'd still have to respect the 79" height to the center of the last row of regular breakers (which are the top with the main on the bottom.)