Is it ok to help pay off my parents mortgage which is not an islamic mortgage but an interest based one. They would find it diffficult to change to an islamic one because of the lump sum fee you have to pay to the islamic company first.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

It is permissible for you to pay off the mortgage of your parents even if it involved Ribaa (interest and/or usury) since they cannot avoid paying the interests thereof; doing so is being kind and dutiful to your parents. However, if it is possible to avoid paying Ribaa, then you are only permitted to pay the original debt without paying the interests.