The field descr would presumably hold the values I originally had in the field Taxon - e.g. the names of various taxons, like 'Mammalia' and 'Canis-lupus.' The field id is just a numerical key, and I already have the numerical values for parentid.
But what am I supposed to put in the field taxonid?
Hm, you might want to take a step back. What does the parentid field you already have represent. Is it you mysql version of the data. Does it point to an id of a taxon from the version? If so you need to redefine the taxon table to just integer, not serial and load that table from you csv dump of mysql.
Otherwise I suspect you will need redo your parentids.