No problem. Just think of o2 as a reseller of RIM software. If you buy your BES server and/or upgrades and/or CALs via them, it makes no odds. They are RIM products, not recoded or changed by o2, and will work the same regardless of whether o2 is your carrier, or whether you change.
Obviously o2 handhelds ARE locked to o2 by default, but we all know the BB devices are a different kettle of fish.
Software and CALs are generic, regardless of reseller. OK, well not TOTALLY generic, a Domino BES won't work with Exchange etc., but you get the point here