This section...
' Set TMR0 to interrupt every 16.384 milliseconds
OPTION_REG = $55 ' Set TMR0 configuration and enable PORTB pullups
INTCON = $a0 ' Enable TMR0 interrupts
On Interrupt Goto tickint

...sets the interrupt frequency, based on a clock timer. When you change the clock speed, the interrupts still happen at the same number of clock cycles. The multicalc program that Mr. E has will help you calculate how often it is going now (documented at 16mS), and help you get there with the clock * 5.