How does one permanently set illustrator to save at its fine drawing state and go to the 72 dpi state on those rare occasions one like me wants low resolution instead of fine drawings?
To what default are you referring? The Save for Web and Devices default of 72 ppi for JPEG and PNG, the Document Raster Effects default or something else. AI files in themselves have no default resolution.
When I open all my old illustrator drawings with the new illustrator and try to save them there is a warning that says it will be saved at less than 72 pixels per inch, that would turn all my old drawings into something as rough as a giff.
That setting only affects pixel based effects - have you used any?
You can change it in the Document raster effects settings.
When creating a new document, watch out for the setting. For documents using the print document profile, the resolution should already be set to 72 ppi.
How old are the illustrations in question? As Monica said it only really affects Effects which use Photoshop Effects (bottom half of the Effects menu) or Blending modes and Drop Shadows.
When I open an illustrator 10.2 document with illustrator CS5 and then save the new document there is a warning that the new document is 72 ppi or less. If I copy and paste the old drawing onto a new illustrator CS5 file it behaves normally. It seems very inefficient to go through years of drawings and copy and paste all the layers into new documents but I guess if that's the only way I will just gradually do it.
Change the Document Raster Effects Settings in the documents as you open them...
Effects > Document Raster Effects Setting
There's no need to copy to a new document. You simply need to correct the setting.
There is a way now, for new documents you create. Use a "print" profile when you create a new document.
Your issue is due to the opening of old documents that were set incorrectly. There's no way to tell old documents to change unless you open them. Originally when you created the old documents, the settings were wrong. Since Illustrator has gorwn up a bit, it's now showing you things are set incorrectly, previously you simply didn't notice the incorrect setting (but it was still wrong just the same).