Amendments to the application, including joinder, substitution, or discontinuance as to parties, may be allowed by the court at any time upon terms that are just and reasonable, but all amendments shall be in writing and signed and verified like the original application. A person who acquires the original applicant’s estate in all or part of the land by voluntary instrument from the original applicant may be substituted for the applicant as to the land acquired by executing and filing an amendment without payment of the fees specified in section 357.021, subdivision 2, clause (1).