Yes you can buy new software and upgrade it.There were some older ksu's that couldn't be upgraded it depended on the revision level of the cabinet which should be on the side of the cover.I think the latest software for the 42/84 was 8.2.4.M phones were introduced before that I can't remember which level might have been 4.?
If you have really old softwrae like 1.12 you need a kit that also includes ram as well as the new eproms.