Changes before starting are very different. After the first international segment things "calm down".

Be aware that the OW Explorer rules do have a fee (used to be US$125) for routing changes. Date and flight number changes using the existing routing should be "free". Changes of transits to stopovers are normally routing changes.

Change are normally made by the airline which has issued the ticket. Also note that some airlines will have an additional service fee as well.

Happy wandering

Fred