I know this is an old topic, but from an old Pioneer Mill report on the history of the Pioneer railway and loco fleet the three Perry's were converted to 0-4-2 as they were "playing havoc with the track".
Excerpt from the report follows:
"Pioneer No.2 or 'Tiapan' and Kilrie were 15 ton, 2 wheel coupled, coal fired Perry locos bought second hand in 1949 from Mount Morgan. Neither of these Perrys had a bogie under the cab when put on the run. They developed a swinging, pitching motion which played havoc with the lines. The bogie from the small Kilrie loco was put under Kilrie and a new bogie was made for Pioneer No.2. One of our retired drivers advises that the 16 ton Klondyke loco also had to have a bogie made for it."
Hope this adds a bit more to the reasons behind the conversions.