The BA B737-436s were bespoke ordered and paid for. They are surely paid off, whereas the WW fleets is leased. So they are (re)introducing an old subtype with fewer seats that is going to need a fair amount of cabin work to come up to BA standards, unless they are going to keep WW seating and interiors.
There are other factors though. I thought read somewhere that the 734s are coming up for major checks - a significant expense for aircraft at 20-odd years old. There may also be significant costs to get out of the leases of the 733s, so despite any interior work needed the case for keeping the 733s and ditching the 734s may be closer than appears at first sight - especially if routes can be arranged to maximise load factors on the 733s. Indeed, I've heard some comments that the 734s need interior work anyway (from a pax perspective) as they are "tired".
MD