You should upgrade to Confluence 3.5 before upgrading to 4.0. See this upgrade notes http://confluence.atlassian.com/display/DOC/Confluence+4.0+Upgrade+Notes#Confluence40UpgradeNotes-UpgradeProcedure
Changing from 32-bit to 64-bit platform should not be a problem. If you are installing Confluence as a service, take note of the following article http://confluence.atlassian.com/display/CONFKB/Problems+Installing+Confluence+as+a+Service+on+Windows+64bit
Agree with Dave - but in addition, you should positively promote it too with knowledge. Power in knowledge, so equip your users.
Share these links and videos with them - or at least be well-versed on them yourself at a very very very minimum.
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