Psychologically, that makes colonists rather extreme individuals, does it not? I mean settling a new world means altering yourself to become a new species, genetically incompatible with other humans and NEVER able to return to your planet of birth ... for both you and your children and their children, forever.
That is a very different mindset than the classic wander-lust to see what lies beyond the next hill, or the basic desire to get a new start in a new land with new opportunities for you and your family.
That would seem to me more akin to starting a colony with Jihadists or members of a cult. Those radically committed to an extreme view directly opposed to the majority of society. I can't even imagine the results of inbreeding populations with a natural pre-disposition for extremism.
In my campaign, it is assumed that the adaptation of settlers to their new environment occurs gradually over a period of several generations - first-generation colonists don't often engage in radical self-modification that makes it impossible for them to return to their homeworld! The gradual fragmentation of the human species into multiple humanoid clades occurs over a period of centuries. This is a long time from the perpective of the human colonists, but is a short period in evolutionary terms.
Incidentally, the notion that the Human species would adapt itself to different environments has a long pedigree in SF circles - I believe that James Blish and Cordwainer Smith proposed the idea of patropy back in the 1950s. So it can be treated as an established genre trope, albeit one that doesn't get a huge amount of attention.
To some extent, my own homebrew campaign assumes that the transhumanist notion of morphological freedom is widely recognised - the idea that people have an natural right to modify their physical form through technologies such as genetic engineering, mind uploading, cybernetics, nanotechnology, etc. I presume that such modifications will become common as the technologies necessary to support them are perfected. To get an idea of what such a future might be like, read Greg Egan's Diaspora or Richard K. Morgan's Altered Carbon.
Because I don't want a full-blown transhumanist future, I do tend to tone down the implications of this technology a bit - stuff like re-sleeving in Altered Carbon is technically feasible, but is not common due to the expense involved.