Unless you have shut off valves or there are still some pipes that are old there should be no need to "have" to put it access to these pipes.
If you do need to then there are several access door kits you can buy at Lowes or H.Depot cheaply and installing them is as easy as cutting the new drywall and pushing them into the hole.
Before you go to the trouble of removing the lathing check to see the thickness of the plaster, you can get easily three different thicknesses in drywall from 1/4" to 1/2" to 5/8" so you might find that putting drywall directly ontop of the lath strips then some minor mudding (drywall compound) brings you to the right finish.
Removing the lath strips is not as easy and often will create a bigger mess before your done.
Hope this helps
Good Luck
