75. Applications to vary marriage settlements shall be made by petition filed after but within one calendar month of decree absolute unless such time is extended by a Judge on summons personally served on the husband or wife as the case may be, the trustees of the settlements and such other persons as the Judge shall direct. Subsequent pleadings shall be as in proceedings for maintenance. Appearance must be entered in the principal cause before an answer is filed. The hearing and investigation of every application to vary shall be conducted by the Judge in the manner prescribed in rule 73 (1) with regard to applications for maintenance.