Greekrage is right on both counts (it will break, if it didn't your changes wouldn't be visible). If you want the details why:
It will break precombineds. This is because the game compares the Version Control Info 1 (VCI1) value (in the record header, visible in xEdit) of all [placed objects] (i.e. REFRs listed in XCRI of the winning copy of the CELL*), to the PCMB timestamp in CELL record. If the VCI1 of any REFR is newer than the PCMB timestamp of the winning CELL record, both precombineds and previs are broken. Same if the VCI1 shows as "None" in xEdit (which will happen if you copy them using xEdit**).
Technically, you could copy that VCI1 data in xEdit using a script, or use the "Update Timestamps" option in the CK to set a new PCMB (and VISI) date, but if you do so without regenerating precombineds and previs, then your changes won't show up in game. This is because a record being in XCRI tells the game to not actually load the REFR, instead relying on the precombined nif file. That nif file has things baked into it, including model (mesh), position (and orientation) in the WRLD (or in the CELL for an interior), scale of objects, and material applied to them.
*Fun fact: it looks at the winning copy of ALL REFRs in the winning XCRI. If your mod rebuilds precombineds, and it doesn't touch some of those REFRs but another mod does, the game will compare the VCI1 of the winning version of the REFR from that other mod (even if your mod is loaded at the end of your load order). If it is newer than when you rebuilt precombineds, precombineds and previs go poof. This makes sense when you consider how the game handles overrides°, but means that it is a pain to track down which mod is breaking precombineds and previs, if you notice they are breaking in-game and it doesn't happen with just your mod and the base game+DLC. Someone is working on an xEdit script to help, but it is not yet published. Once it is, I will 100% be referencing it on my mod page, as I am sure some others will
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**This is intentional, as back when xEdit copied it (back in TES4 days), it caused problems for people using the Version Control system in the CK, and until FO4 it never mattered for anything else.
°Once loaded, the game doesn't distinguish which plugin a winning override came from. It just discards all non-winning versions of records, with very few exceptions (REFRs and CELLs are not one of those exceptions).