Shortly, the new version of Google Sheets will be rolled out to all users, and will from that date be used by default when creating a new spreadsheet. If you still have spreadsheets in the old format, they will remain in that format for the moment, but will eventually be converted automatically to the new Sheets without any user action.
If you want to manually convert these sheets to the new format, however, you can do so.
In the old version spreadsheet, click the arrow next to a sheet tab and select Copy to… to copy the sheet and all of its contents to a spreadsheet created using the new version.
Alternatively, you can export the contents from the old version and import them into a sheet created in the new version.
If you share an old-version spreadsheet, your colleagues will see the content in a spreadsheet of the old version, even if they have opted to use the new Google Sheets.
Don’t forget that your old sheets will eventually be converted automatically, so you only need to consider thee actions if you need your sheet converted immediately to the new format.