You can switch back and forth, no problem. That is an old wives tale from the 80s.
You can go longer intervals if you want. On a previous vehicle I went 10K for changes, but changed the filter every 5K. Seemed to work well. Once my DD is out of warranty I will probably do that again.
Of course, those of you burning oil don't really need to change it at all, the new oil you are adding refreshes the additive package regularly.Seriously.