Does PAKLEA replace UltraScape 10?

No. UltraScape 10 is the standard first cart. PAKLEA is the estate cart when the LEA amp is part of the spec.

Can I add PAKLEA pieces a la carte?

You can buy AS-2, subs, and amps separately. The PAKLEA SKUs exist so the estate combination is one line on the PO.

When to use a PAKLEA estate package

Three live packages: six AS-2s with a 10-inch burial sub and LEA 352; four sats plus a 12-inch burial sub and LEA; four AS-3s plus a hardscape 12 and LEA. Standard UltraScape 10/12 still ship without an amp.