1) Do not disengage the automatics (AP/FD/ATHR).
2) activate SEC FPLN (if it is already programmed with new RWY), otherwise change RWY in the primary FPLN (recommended).
3) aircraft will automatically turn towards and intercept the new LOC and it will pitch UP/DN to maintain the new GS.
4) FMA will keep showing GS & LOC.
5) GPWS will not activate as the new GS is close the old GS.
I found the aircraft (320/A330/A340) does it much better if it is taken by HDG then rearming APPR (follow new GS by controlling VS until fully established). Thanks
WTF?
AP off, FD off, Bird on and activate the secondary.
Simples.