While the action is 'valid' (not quite sure what you mean by that actually... it is a possible course of action), it may have some complications, depending on your situation.
a) You can't open a bank account in Japan, unless you have residence status - ie, you are on a student, work, or cultural visa.
b) You can only close your DBS MCA account in person, over the counter - It can't be done remotely, over the phone, or by internet.
If you're moving to Japan, and will be here for a while, then (a) is not a problem, but (b) might be. If you're not moving to Japan, then (a) is impossible, and (b) won't be a problem instead.
Some banks in Japan set the ATM withdrawal limit for foreign cards at JPY50,000 per transaction. So, you'll need at least 6 withdrawals to pull most of your JPY280,000 out. You might like to work out the fees involved in doing so, and figure out if it would otherwise be cheaper to just TT the funds into your JP account instead, if you already have one setup at this time.
If you're moving to JP, it could be a little difficult to get a credit card here. It's not impossible, but it could take some time. In such a case, simply keeping your DBS MCA, and using it for debit only functions is perhaps another course of action you could consider, since there are no fees involved when you do so. The upside of doing so is, if you'll be here for a while, you can always 'top up' your JPY wallet from your SGD balance via internet banking when yen is weak, as required.
The 'buy/sell' spread is the difference in the bank rates when you buy, or sell the currency.
At this point in time (rates at 30/04/2018, 7:03pm) DBS rates are:
JPY100 Sell 1.2273
JPY100 Buy 1.1981
(Rate at: 01/05/2018, 11:48am)
If you were changing SGD to JPY, and want JPY100,000, the bank uses the 'sell' rate, and it'll cost you SGD1227.30
If you were changing JPY to SGD, and had JPY100,000, the bank uses the 'buy' rate, and will only give you SGD1198.10 for it.
So, if you changed SGD > JPY > SGD immediately, you would effectively lose some money, because of the difference in the banks 'buy' and 'sell' price (which is where the bank makes money on FX).