How can I use old projects that were created in AE5.5?
I'd love not have to rebuild them.
You need to open them in an intermediate version and save them. Download the trial version of e.g. AE CS4 and install it.
Download CS4 products
Mylenium is correct.
See this for details of which version of After Effects can open projects from each older version:
opening After Effects projects from previous versions and saving back to previous versions
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