lastpass import csv number format goes exponential

If you postprocess a CSV file with Excel, you get all the automated processing of Excel, which includes autoformat for numeric values. For Excel, a field with a numeric value contains a numeric value. It doesn't know that it's actually a userid string that must not be formatted according to numeric formatting rules. That's standard Excel behavior.

You have better control over how Excel treats csv fields by not double-clicking a csv file in Explorer and let Excel open it, but instead start Excel, then go to the "Data" Category and choose "From Text" in the external data section. This starts the csv importer wizard, and in the last wizard screen you can define the data types for each column. For the userid field that might contain numeric values but should actually been treated as text, choose "Text" instead of the default "Standard" column type. This will do no (or less) undesired autoformat of numeric values in that column.

You might also try the converter from MrC and not edit the export with Excel: https://1password.community/discussion/101693/moving-to-1password-from-another-password-manager I didn't try it with Lastpass, but it might create a better conversion of the csv export than the default importer - MrC tries painstakingly to reproduce the original data to fit the various 1Password item types.