What version of XP Pro is your upgrade CD? I think if you can do a clean install of XP with it when the installer begins, as I recall it will just ask for the Win 98 CD to be inserted at some point during the install to confirm ownership. I do not remember exactly since I only upgraded one '98 machine years ago.
If it is an older version of XP you can slipstream the CD into XP SP2/3 with nLite, but you need a computer running Windows to do this. There are manual methods to slipstream too I would try googling it. However the only methods I know require Windows to be installed.
What version of XP Pro is your upgrade CD? I think if you can do a clean install of XP with it when the installer begins, as I recall it will just ask for the Win 98 CD to be inserted at some point during the install to confirm ownership.