Hmmm, simple replacement it should be, but you would still need to lower the PIR to a level of say 1400mm AFFL, which means you'll have alarm cable coming down the wall.
Not sure on the "pet distingiushing" PIRs, but if the cat moves fast enough then I presume it's signature could be mistaken for a larger object and hence set the alarm off.

I've only ever seen one cat friendly alarm in a house with two cats, and that was where there was no PIR's and all window and door contacts.

We too have cats.

Most alarms now have zoning.

Each night I 'part set' the system disarming all the sensors except the contact switches on the front and rear door and the garage PIR.

Peace of mind...

Delooney