Well, as you probably already know, the whole cell phone system in Japan is totally different to the rest of the world. As far as i saw when i was there, they've only really just started with *some* makes of phone being sim card capable. As for having the same number...i think its going to be pretty much impossible unless you want to pay a whoooole lot of money for it and then a whoooole lot more for the monthly bills. I think the best thing is to get a new one. Im pretty sure vodafone do some good 3G sim phones (which i think you can use when you get home). Best thing to do is to go to some english friendly shops when you get there (or if you have one, a Japanese friend is just as good). The shops where you usually find they have english speaking staff are mostly in the big cities (im not quite sure where you're going...) If you're going to Tokyo, Shinjuku, Shibuya and i think Akiba are known for them. Also the businessy areas have english speakers due to...well...ya know, the businessy english folk (there was one right next to my work!) How long are you going to be in Japan for? Cause, if you are going to be there for a year, buying a phone there is uber cheap. I got mine for....1yen? and mine was a fancy pantsy does everything phone (however, not 3G...argh!) and i paid like, 6000yen a month. Even if you're only going to be there for say...3-6 months, its worth going on a plan. From what i remember, the contracts for phones usually go for a year and if you cancel before that year is up you pay around 4000-5000 yen for the cancellation fee (you dont pay anything if you cancel a year to the month when you bought it). Most phones in Japan (unless they're a gazillion years old) can be customised in every shape/size/ringtone/form. I had mine in pink ^__^v. You can dl ringtones, stick on little glittery things (phone decoration is BIG there man...some of the jewels i saw on some of the phones..O.O) I think you can also dl japanese to english and vice versa dictionaries to your phone. I know a friend of mine did that (she was on vodafone) and it was pretty good as it was always being updated with new words/slang etc (very useful when she was trying to chat up cute j-boys...lol) You can also arrange it so that your bill can come off either a) your credit card or b) you can get it sent to your address and then you can go to a convenience store and pay for it (most convini's do it)
Ok, i hope i've helped...or made some kind of sense...or something heh! If i havent, sorry for wasting reading time @_@. If you want to ask anything feel free and i'll see if i remember any other phone info ^_^