Microcomponent maintenance microbot
Diamondoid AP products made out of microcomponents can potentially be taken apart again to do self repair or system upgrades. If the exchange of microcomponents is done while the system is not active it's microcomponents can be disassembled by the upper assembly levels of a compatible nanofactory but if the exchange shall be done while the system is running or without disassembling the whole system mechanisms for transport of microcomponents must be present.
Microcomponent maintainance units - small diamondoid AP products out of a single or multiple microcomponents - featuring legged mobility would be a possibility to provide this mobility. They could e.g. hold onto a simple cubic crystal lattice with eight arms and move from interstitial point to interstitial point by arm extension or shrinkage. Further manipulators would allow them to unlock remove and transport microcompoments. For high throughputs a fractal channel design will be necessary but for self repair of usually low damage rates a simple cartesian channel system might suffice.
Note that microcomponent maintainance units do not have to have such high "intelligence" (fluid dynamics emulation) like (speculative) utility fog has to have. Also they have no means for self replication like the deprecated concept of molecular assemblers.