You can just drag and drop your tiddlers into your new TW file.
That's the good news. The bad news is that the formatting codes changed between versions. Among other things, carriage returns no longer inform the end of a paragraph. Sections/slices are history. Plugins/macros for the old system don't work in the new.
What I've done is to drag and drop tiddlers. Then as I actually needed them, I would enclose their contents in triple single quotes in order to prevent the paragraphs from flowing together. Later, when I have more time, I can change the formatting to the new system, when possible.
Good luck
-- Mark