Redmine

Actually, there's nothing to do to upgrade old projects. It seems to be a bug and I can't reproduce it.
Does this problem occur with any url which contains an old project identifier ?

Could you give an example of an url that gives a 404 (with a real project identifier) and one that works.
Also, give me your exact database and rails versions please.

my very first project (id 1, identifier test1) still works, none of the others which are all
subjects of the second project work. Using the id is fine.
http://red.rahu.loc/projects/show/4004 404 The page ...doesn't exist or has been removed.
http://red.rahu.loc/projects/show/12 works, click on news to get served
http://red.rahu.loc/projects/4004/news error 404 as above, but if I enter
http://red.rahu.loc/projects/12/news looks like it should

new project being a helpdesk with an identifier cdhelp & id 17
http://red.rahu.loc/projects/show/cdhelp works fine

Admin info says 0.7.0 (MySql), rails version 2.0.2

The problem is that your identifiers shouldn't be numeric. They should at least contain a letter.
http://red.rahu.loc/projects/show/4004 => will look for the project with id 4004
http://red.rahu.loc/projects/show/test1 => will look for the project with identifier test1

I'll add a validation rule to ensure that projects can not be created with a numeric identifier.
In your case, the only way to fix it is to manually change the project identifiers in your database (eg. replace 4004 with p4004 or anything with at least a letter). Of course, don't change their ids.