You can replace the serial number in the old records but first you must perform a find to constrain the found set to only the old ones. Otherwise this will update all records including the new ones. It is irreversible so make a backup first.
Edit the key field (File>Manage>Database…) and find the next serial value on the Auto-Enter tab.
Select the key field in Browse mode and choose 'Replace Field Contents…' from the Records menu. Select the 'Replace with Serial Numbers' option and set the Initial Value to the next serial value so that there is no overlap with the existing serial values. Also check the Update serial number in Entry Options to reset the next value in the Options for Field dialog box. Finally click the Replace button