Yeah, DBZ's gone through Hell concerning dubs, especially over the last few years.
FUNimation originally began dubbing the show using the Ocean voice group, which is based out of Vancouver (Canada, obviously). They dubbed the first 13 episodes of "DragonBall," 53 episodes of DBZ, and three DBZ movies with this group.
After these two seasons, Saban (their distributer) backed out for good, leaving them with pretty much NOTHING, including no money to continue production with an outside voice group (oh, how the money situation would change... ;_;).
FUNimation began, with episode 54 (beginning of season three) dubbing the episodes in-house with local voice talent. This meant an ENTIRELY new cast. The point, at first, was to attempt to mimic the Ocean cast as best they could. Later on, though, most of the voice actors / actresses have really gotten into the characters, and can claim them as their own.
So FUNi's been doing that ever since.
Somehow, though, this company called KidBiz got the license to do ANOTHER English dub (based out of England, I think?). KidBiz went back and hired the Ocean cast to do the voices, so MANY of the characters got their "original" voice actors back. They've been doing this dub from most of the third season onwards to the end, now.
HOW this dub can air in Canada I'm not sure. Technically speaking, FUNimation has exclusive rights to all DB merchandise and broadcasting in North American and Australia. There's obviously some kind of loophole that KidBiz was able to get around. This is also why FUNimation can't put both dubs on their DVDs; technically, they don't own the rights to the KidBiz dub. I'm pretty sure that in Australia, though, MadMan Ent. is putting out FUNimation's DVDs, and someone's putting out the KidBiz episodes, as well...
As for poor Son Gokuu...
In DBZ he started out with Ian Corlett (who most people will know as Dr. Tofu in Ranma). Around episode 35, Gokuu was switched to Peter Kelemis (who's done voices on Ed, Edd, and Eddy). Peter did Gokuu up until the end of the Saban run with season two, as well as the first three DBZ movies. When FUNimation took over by themselves with season three, local actor Sean Schemmel took over as the voice of Gokuu, and continues to do the voice.
The new KidBiz dub, though... Peter Kelemis took back Gokuu for this dub for a decent run, and got through at least into some part of the Jinzoningen era, I believe... at which point Kirby Morrow (Trowa in Gundam Wing, Cyclops in X-Men Evolution, etc) took over the role.
Ian Corlett left during the second season because.. well.. he just wasn't getting paid enough. I'm not sure if he continues to do much voice work, but I do know he works behind the scenes on script-writing, still. Kelemis... uhh... I have no idea why he stopped. Probably got too many complaints ;_;. As far as I know, Morrow is still doing Gokuu in the KidBiz dub.
The big question for KidBiz is... why? The only solution fans have been able to really come up with is that KidBiz is ripping off FUNimation SO much (the scripts are nearly IDENTICAL... most absolutely verbatim) and churning out crap at such a fast rate, that stations like YTV are able to pay a smaller amount and get MORE material to air than waiting for FUNimation.
On a much larger scale, yeah, it sucks that dubs sometimes have to do through voice changes. I know in the Kenshin dub Soujiro had a voice change about mid-way through the Kyoto arc (I was kinda fond of the first one). There's always Zelgadis in Slayers (forgot the first guy's name, the second is Freeman). It happens, but ah, well.. it even happens in Japanese shows. Both Kamesennin and Chi-Chi had two different seiyuu in DB/DBZ (I think Kamesennin's actually passed away, at some point near the Buu era).
Back to work... *sigh*... hehe :D