This page summarizes the general techniques for performing a taxonomic split in Avisys. There are two kinds of split; one where a species is split into entirely new species and the old species no longer exists, and one where part of a species is split into a new species but the old species still exists. We'll use the split of Mew Gull into Short-billed Gull and Common Gull as an example of the first kind of split, and the split of Brown-headed Nuthatch into Bahama Nuthatch and Brown-headed Nuthatch as as example of the second kind of split.
Of course, you don't need to do anything at all if you have no sightings of the split species, so we'll assume here that you have sightings of Mew Gull and Bahama Nuthatch.
You only need to prepare in advance for the first kind of split, so here we'll only talk about Mew Gull. First, rename the species to one of the split species. If you have sightings of both of the split species, never mind that until later; for now just rename the species to the split species for which you have the greatest number of sightings. So, if most or all of your Mew Gull sightings are in the range of Short-billed Gull, rename the species to Short-billed Gull. To rename a species, highlight its name (by clicking on it), click on Edit->Edit Species/Family Name, change the name, and click OK.
For the Brown-headed Nuthatch/Bahama Nuthatch type of split, there is nothing to prepare in advance.
How to do this is described here.
If you have only seen Short-billed Gull but not Common Gull, you are done with that split now. Likewise if you have only seen Brown-headed Nuthatch but not Bahama Nuthatch, you are done with that split. If you have seen both of the split species, you have a little more work to do.
First, find Common Gull and click on the name to highlight it. Then, either press Ctrl-F3, or equivalently click Utilities->Save Species for Edit. Either way, this will bring up a "Saved Species" dialog listing Common Gull. Just click OK here. Then do a List Records of all of your sightings of Short-billed Gull in the range of Common Gull. Change each of these records to Common Gull. To do this, click on the first sighting and then click on Edit, and then on Species. Clicking on Species will bring up the Saved Species dialog that you saw before, and you then click on Common Gull, and then on OK and Finished. You have now changed the species associated with this sighting from Short-billed Gull to Common Gull. If you have a second sighting, highlight it and press enter, and then choose Yes for the question "Do you want to do mass modifys?", and then simply press enter on each subsequent sighting to perform the rename. Repeat this process for each relevant Place. The whole process is simpler than it sounds.
For the Bahama Nuthatch type of split, the process is similar. Do a List Records showing all of your Brown-headed Nuthatch sightings that should be changed to Bahama Nuthatch (e.g., select by Place) and move them all to Bahama Nuthatch using the same procedure as described above.