Altering open source fonts?

See clause 2 of the OFL:

2) Original or Modified Versions of the Font Software may be bundled,
redistributed and/or sold with any software, provided that each copy
contains the above copyright notice and this license. These can be
included either as stand-alone text files, human-readable headers or
in the appropriate machine-readable metadata fields within text or
binary files as long as those fields can be easily viewed by the user.

So you must include the original copyright notice, and not replace it with your own. If your fonts include novel content that is not derived from the existing fonts, then you could add a second copyright notice indicating that parts of the font are your original work.

If you were combining multiple OFL fonts, or parts thereof, into a single modified font, then you would need to include all of the relevant copyright notices.

With regard to Designer metadata, this is not a legal requirement for inclusion in any font, but it is polite to retain reference to the original designer(s) as well a providing useful information to users. So, for example, you might say ‘Designed by X, based on prior work by Y’.

I was thinking in terms of incorporating some rather than all of the glyphs from one libre font into another. When merging glyphs from multiple fonts into a single font, it is necessary to include the relevant copyright notices for all the sources, even if only a single glyph is taken from a particular font, hence ‘parts thereof’.