Looking at the ring of spell knowledge it says:
Through study, the wearer can gain the knowledge of a single spell in addition to those allotted by her class and level. A ring of spell knowledge I can hold 1st-level spells only, a ring of spell knowledge II 1st- or 2nd-level spells, a ring of spell knowledge III spells of 3rd level or lower, and a ring of spell knowledge IV up to 4th-level spells.
A ring of spell knowledge is only a storage space; the wearer must still encounter a written, active, or cast version of the spell and succeed at a DC 20 Spellcraft check to teach the spell to the ring. Thereafter, the arcane spellcaster may cast the spell as though she knew the spell and it appeared on her class’ spell list.
Arcane spells that do not appear on the wearer’s class list are treated as one level higher for all purposes (storage and casting).
I'm interested in knowing whether or not you can change the spell stored in the ring after a spell has already been put in it. My gut instinct is no since there isn't anything specifying how long it takes to teach the spell to the ring.