So, a player wanted to have a more robust familiar, and suggested a feat. One of the options of the feat was a "Maturing Familiar"... at higher levels, you could choose stronger forms for your familiar. I like the general idea, but wanted to make it available without a feat, so I've worked up this.

Find Familiar can be upcast with a 3rd, 5th, 7th, or 9th level slot.
3rd - CR up to 1/4
5th - CR up to 1/2
7th - CR up to 1
9th - CR up to 2

They retain all abilities of their type, plus the abilities of a familiar. Your familiar remains in that shape until
1) It is killed
2) It is permanently dismissed
3) You cast Find Familiar again to change its shape.

Each time you cast find familiar, you must use a slot appropriate to the form you are summoning (or, if casting as a ritual, choose any of the basic forms).