Um, if you have the ability to arbitrary rearrange your cells, they're likely more loosely attached to each other than those in animals. In any case you should be able to process raw environmental material to grow.

I have thought about possible future societies that aren't based on biological reproduction. Most of the examples in scifi fall apart after a little MoR-style pocking (their main flaw is keeping certain aspects of the setting the same as ours even if those aspects no longer make sense). The two that seem stable are the following:

1) Sentients become pure infomorphs (or ems as Hanson calls them). This abolishes the distinction between memetic and genetic inheritance. Thus "sex" in the sense of recombining "genetic" material is separate from reproduction and resembles having conversations. Reproduction consists of creating a copy of your mind/source code. This resembles the life cycle of bacteria, who exchange information via conjugation but reproduce via binary fission.

2) Sentients are produced in centralized factories, e.g., most children are grown in artificial wombs, or robots produced in more conventional factories. This effectively makes them eusocial with all that implies about their sense of individuality, or rather lack thereof.

The other possibility is old-fashioned sexual reproduction. I have no idea which of those possibilities will come to dominate.