OK to mix new 722.9 (3353) fluid with old 722.6 fluid during trans service?

[irieite]

On my low mileage '00 c280, I plan on just dropping the pan, replacing the filter, adding a magnet and topping off the fluid this weekend. Unfortunately I have no converter drain plug.. I understand that the new 3353 fluid is backwards compatible with older transmissions, just don't know about the mix.

My question is would it be a good, bad or OK idea to top it off with the new 722.9 (3353) fluid or stick with the older spec fluid?

Bonus question: Does it matter where I stick the magnet in the pan? I understand there are electronically controlled solenoids nearby on the valve body..