This looks like a good opportunity to save costs and simplify trademark portfolios. However, such seniority expires if the national mark lapses, with effect before the filing (or priority) date of the European trademark (article 39 of Regulation [EU] 2017/1001).
These provisions should be read in conjunction with article 6 of the new Directive (EU) 2015/2046. They provide that where the seniority of a national trademark is claimed, and when that national trademark is not subsequently maintained, the invalidity or revocation of the national trademark can be decided after the trademark was abandoned, provided that such invalidity or revocation could have been declared at the time the trademark lapsed.