XLS to XLSX — the same data in a different container

Replacing .xls with .xlsx is a container change, not a data change. The reader walks the BIFF records, the writer builds sheet XML, and the cells that survive are values and sheet names. Everything the old format stored about appearance stays in the old file.

Cell formatting, colours, column widths and merged ranges are dropped; only stored values survive.

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What survives the rebuild

Sheet names, in workbook order, sanitised to characters Excel allows.
Cell values, including cached formula results.
Displayed date text, so a formatted date keeps its appearance as text.
Empty cells inside a row, preserving column positions.

Questions

Is this an upgrade of the file?

It is a container change. The values move; formatting, widths and macros do not. Treat it as a data transfer, not a modernisation.

Why is the .xlsx file bigger?

Sheet XML and a shared string table are verbose compared with packed binary records. Size growth is expected and does not mean extra data.

Do formulas keep working?

No. Only cached values are written. Re-enter formulas in Excel if they must recalculate.

Are sheet names kept?

Yes, in workbook order, with characters Excel forbids replaced by underscores and names shortened to 31 characters.

What does not survive

Cell colours, fonts, borders and number formats are left behind, along with column widths, merged ranges, defined names and macro streams. Formula definitions are also dropped: only their cached values are written.

How the new workbook is built

Each worksheet becomes a sheet part inside a compressed package, with a shared string table for text. The result is usually larger than the original binary file, because XML is verbose where the old format packed records tightly. Size is not a signal of data loss here.

Formula honesty

A converted workbook shows the values Excel last computed. It does not recompute, so a workbook saved with a stale result keeps that stale result.
Why cached values are reported

If you need live formulas

Re-enter them in Excel after the conversion. The converter deliberately drops formula text rather than copying it, because copying definitions without a calculation engine would produce cells that claim to be formulas while nothing has verified them.

Sheet selection matters

With every sheet selected, sheet names are preserved, which is the only route where multi-sheet structure is meaningful.

Option	Workbook produced
First sheet	one worksheet
Named sheet	the chosen worksheet
Every sheet	all worksheets, rebuilt in workbook order

Checking the rebuilt workbook

Convert the .xls file and download the .xlsx.
Compare sheet names with the original workbook.
Spot-check a date column and a numeric column.
Re-enter formulas that must stay live.
Apply formatting again, since appearance was not carried over.

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