Regarding your specific problem I suggest treating each existing dated folder of image files as a single Project in Aperture. In Aperture a Project is a specific time-based concept that may or may not jive with what you previously considered a project. For instance I may have in my mind that shooting all the highest peaks in every state is a "project," but that would be inappropriate as an Aperture Project. Instead each peak might be a Project, but the pix of all the peaks would be pointed to as an Album.
The way I look at it conceptually:
Aperture is a database (DB), and each image file lives in one Project.
Albums are just collections of Pointers that point to individual image files living in one or more Projects. Since they just contain pointers, albums can be created or deleted at will without affecting image files. Very powerful. And Albums of pointers take up almost zero space, so they are fast and do not make the Library size grow.
Keywords can be applied to every image separately or in batches. Keywords are hugely powerful and largely obviate the need for folders. Not that we should never use folders, just that we should use folders only when useful organizationally - - after first determining that using keywords and albums is not a better approach.
As one example imagine the keyword "flowers." Every image of a 100,000 images Library that has some flowers in it has the keyword flowers. Then say we want to put flowers in an ad, or as background for a show of some kind, or to print pix for a party, or even just to look for an image for some other reason. We can find every flower image in a 100k-image database in 2 seconds, and in another few seconds create an Album called "Flowers" that points to all of those individual images.
Similarly all family pix can have a keyword "family" and all work pix can have a key word "work." Each individual pic may have any number of keywords. Such pic characteristics (work, family, flowers, etc.) should not be organized via folders.
So by using keywords and albums we can have instant access to every image everywhere, very cool. And keywords and albums essentially take up no space in the database.
Another approach is to use a folder "Family" for family pix, a folder "Flowers" for flowers pix and another folder "Work" for work pix. IMO such folders usage is a very poor approach to using an images database (probably stemming from old paper or film work practices). Note that one cannot put an image with family in a field of flowers at a work picnic in all three folders; but it is instant with keywords.
As an aside note that empirically many users find that even though the Aperture DB can contain hundreds of thousands of image files without problem (Referenced-originals Library), individual Projects should be limited in size for speed reasons. Personally I limit Projects to a maximum of ~400 RAW NEF files, but hardware and workflows vary.
For shoots that comprise more than 400 pix I just split into multiple Projects of similar names. E.g.
An Album titled 120707_KJones_Wed would point to all the pix of the 120707_KJones_Wed.