how to transfer iMessages

hi - my mother has bought an iPhone SE to replace her ageing 4S. she would very much like to keep the iMessages from my my late father (he died end of 2015, so still very raw).

along with the new SE, i've also signed her up with a new provider (new SIM, it will be the same phone number).

does anyone know how we can transfer the old texts for her?

thank you in advance,

James.