Beginning from March 2018 Nippon Budokan will be closed for renovations and expansion until June 2020:
http://www.decn.co.jp/?p=92512
This means that after March 2018 there will be no Hello!Project groups holding concerts at Budokan in 2018 and 2019.
The Main Budokan Hall isn't shutting down until September 2019中道場の着工後、19年9月には本館を休館し、老朽化の懸念がある既存の大屋根などの改修工事に取り掛かる。日本武道館の担当者は「基本設計の完了で計画のアウトラインは固まった。計画詳細の公表は実施設計の進ちょくなど時機を見て判断する」と話している。
Of course, that doesn't mean that H!P groups will be playing there. Could be a rush of other people who want to squeeze in before the remodel...