Hi welcome to the forum. Without knowing which box the old one and which the new one is can’t answer the question. You wont be able to transfer HD recordings in HD to a SD box under any circumstances.
Possible HD Humax boxes as the source.
Satellite based
Foxsat-hdr
Terrestrial based
HDR-FOX T2, HD FOX T2 (one other you can’t buy yet the Youview version of the HDR-FOX T2)
Possible SD UK Humax terrestrial pvrs you may have.
8000T, 9200T, 9150T, 9300T.