TD42T glow plugs

[whk173]

Hi I’ve just picked up some replacement NGK glow plugs for my TD42T and found that there’s two different ones listed changing mid ‘99. My engine is 5/99 so the listing is for the slightly longer (about 2.5mm). When I removed the old ones I found that they were the shorter ones. Does anybody know why the change was made and is it critical which plugs go into which engine?
Also something I hadn’t thought of before is should I have swapped the timer unit when I did the ZD - TD conversion? I have noticed they don’t both behave the same.
Thanks