You mean apart from new software coming in due to changes of curriculum? Or new staff coming in with better ideas about things? Funding for particular projects which have different software / system requirements? Staff leaving so the school opts to purchase resources you don't already have? Students (in secondary in particular) coming up from schools where they have been using mobile devices very well and are now expected to just drop that set of skills? Students having more to do with the IT of the IT of the school (Search for SSAT & Digital Leaders project)? A growth in computing and programming (Search for Scratch, Kodu, PHP in schools, Arduino)? A move to games-based learning perhaps (Search for Dawn Hallybone, Tim Ryland, Redbridge game network)? I would have previously said a change in specialism might force through some stuff (eg Arts, Language, Computing, etc) could affect this but recent changes make this unlikely now ... and this is before we get started on schools moving to academies or hard/soft federations, where networks are joined together and you might not have too much choice about dealing with a setup different to your own!
Meanwhile, questions for GD, Spannerwotsit or any other folk who do Authoritative[tm] for a hobby:
1) List some compelling, pragmatic reasons why a system happily running XP, IE8, FF-de-jour or whatever together with a large prior investment in software for that platform, shouldn't keep doing that until at least 2013 (when the next Windows *may* hit the streets). Please skip the old drivers for new h/w issue, we all know about that. Don't forget to include any supporting evidence re. improvement in educational attainment.