Extract the W98 CD content to your hard disk (e.g. F:\W98)with WinISO as a bootable .iso-file
Within this file find: Precopy2.cab
Open it with WinRar
In WinRar, find setupx.dll & extract it to somewhere.
Open extracted setupx.dll using eXescope
In eXescope, expand "String" & find number "96"
Click on "96", on the left you will see this number "1526,111" or "1526, 121" or "1526, 115"... ... The last 2 digits may vary depending on the version you have got. In any case, change "1526,1xx" into "1526,100" and save.
Import the modified setupx.dll with the help of WinISO into the same folder where precopy2.cab (and a bunch of other cab-files) reside. Save and burn this .iso-file with CDRWin.
Now, each time you run Windows 98 setup, you won't have to enter any serial number, nor will there be an upgrade/OEM/full check. You can do either a clean install or an upgrade as you wish, because it became a full version.