How many miles do you have now? And how many travelers for your trip to Japan?

It's worth noting that spring is the high season. Fares will be higher and awards will be harder to find. Definitely not impossible though! Late March will be tougher than early March, as you are getting into the beginning of sakura season.

Also hotels will be more expensive and things will be more crowded in general, as there will be more tourists. The trees are beautiful though, and the weather is usually pretty nice (though if you are unlucky you may get some rain)