geometry package or native registers for mid-document margin changes

Direct answer

Yes: with the geometry package under LaTeX2e, \newgeometry is the supported way to change margins multiple times in one document. It forces a page break and, in twoside mode, recalculates the odd/even offset pair so facing pages stay symmetric. Native register edits are not forbidden, but they must be made before the page is shipped out and you must update both \oddsidemargin and \evensidemargin yourself; dependent lengths such as \textwidth do not adjust automatically. For KOMA-Script classes, do not disable geometry entirely—load it with the KOMA option so it defers to KOMA's layout engine instead of fighting it.

Confirmed behavior vs likely explanation

Confirmed: \newgeometry ends the current page and starts a new one with the new layout. \restoregeometry returns to the preamble layout. geometry recalculates margins, text width, and text height together.

Likely explanation for drift: If you change only \oddsidemargin after a page has begun shipping, the recto and verso pages can receive different offsets. Manual edits also leave \textwidth and related lengths stale unless you recompute them, which geometry does for you.

Steps for this case

1. In the preamble, load geometry once with the target paper size and initial margins: \usepackage[twoside,margin=2.5cm]{geometry}
2. Where the margin set must change, insert \newgeometry{margin=3cm} at a paragraph or section boundary, never in the middle of a page.
3. To return, use \restoregeometry at the next boundary.
4. If you must touch native registers, wrap the change in \clearpage or place it before \shipout, and set both \oddsidemargin and \evensidemargin explicitly.
5. With KOMA-Script (scrbook, scrreprt, scrartcl), load \usepackage[KOMA]{geometry}. Without that option, geometry and KOMA can redefine the same layout lengths and produce unexpected offsets.

Assumptions: LaTeX2e, current geometry release, default output routine. Classes or packages that redefine \shipout may change where a boundary is safe.

Verification

Compile with pdfLaTeX, LuaLaTeX, or XeLaTeX and inspect odd and even pages after each \newgeometry call. The outer margin should mirror on facing pages. If it does not, the change was likely inserted mid-page or mixed with manual register edits.

Missing diagnostic: Are you using a standard LaTeX class (article, report, book) or a KOMA-Script class? That determines whether the KOMA option is required.