You can't change the config bits to switch between LP and XT.
It doesn't work setting config to XT and then plonking in a 32k
crystal and vice versa. The PIC will work with a low frequency on
Oscin when set for XT but won't excite an LP crystal across the
Osc pins

So I'm pretty sure it would have to be a hardware solution under
s/w control. Two circuits I looked at today -

4194304 oscillator driving a 74HC393 binary divider. The
derived 32768 output goes to one input of a NAND. The other
input of the NAND goes to a PIC pin. A logic one coming from
the PIC enables 32768 on the output of the NAND. The original
4194304 is also wired to a NAND gate like this. The two NAND
outputs are connected to the two inputs of a third NAND, the output
of which goes to OScin. By toggling the two PIC pins (or using a
flip-flop)  you can select the PIC's running frequency at 4194304Hz
or 32768Hz. The big problem is the current consumption. If you're
trying to save power forget it. Might be useful for some obscure
timing application which needs two related clocks

Circuit 2 is two Harris (Intersil) HA7210 crystal buffers. These have
a disable pin which puts the output in hi-Z mode so two or more can
be connected to Oscin. Enable one, disable the other(s). Current
consumption is 10uA at 32k and 230uA at 4194304. Much simpler
to implement than the above but not so cheap. Works a treat though
and each can be an independent frequency anywhere between
10kHz and 10MHz

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