Using an External interrupt from any pin PIC16F873A
Several issues:

This is the poster case for why you need to carefully chose which pins of the micro will be used in what ways.  "I'm using them for interfacing to an LCD" is no excuse.  Go use some other pins for the LCD that don't also have a special hardware function you need.  Assign pins that use special hardware functions first, then assign general I/O to whatever pins are left over.

The PIC 16F873A is quite old.  There are probably newer PICs with the same footprint that have a lot more I/O capability, possibly including more interrupt on change pins.

There are other ways to cause a interrupt, such as the INT pin specifically for that purpose, the CCP inputs, and even timer clock inputs if you're clever.  Some of these may use up other hardware just to get the interrupt, but there is no harm in that if you weren't going to use that hardware anyway.

See point one.  Assign the pins properly.  This really should be obvious.