Personally, I prefer, in some cases, to use the attribute manager to "OVERWRITE" (not APPEND) the new layer into the target PLN,
before copy+pasting the objects.
That way, the new layer ends up with same index number in both files, assuming that is, they were both derived from the same or similar template - which would mean that the index number of the new layer in the source PLN is less likely to exist in the target PLN. If it doesn't, then you can use OVERWRITE to add it to the source, which will give it the same index number as the source PLN.
Why this method?
This is useful because LAYER COMBINATIONS are collections of layer index numbers, not layer names, so keeping all your LAYER COMBOs synced between files with the OVERWRITE function in Attribute Manager is easier. If the layers in 2 files have the same index numbers, you can quickly sync the COMBOs in Att.Man. by a SELECT ALL and an OVERWRITE (in the COMBO tab).
Whenever practical, I do the same for other attributes too - fills, composites, materials, etc. This is more important the more hotlinked modules you use, because attributes primarily refer to each other by index number, not name. If the attributes aren't synced by index number, a hotlinked module can end up looking quite different in its source file than in its host.
SIDENOTE: Inconsistently, and annoyingly IMHO, is that VIEWS refer to their LAYER COMBO by name, not index number, so if you change the name of a COMBO, it loses the link with all VIEWS that refer to it.
DISCLAIMER: I'm only an AC junior, and I've only just discovered all this in the last few months. So if you know if I'm wrong in any aspect of my understanding of this, please correct me!