Jerantut, most RTW fares included OW's only require you to fix and not change the first sector (sometimes on certain RTWs the first sector upto leaving the continent). You will need to fix your route though. You can change the dates as and when you please (after the first sector) as long as you meet the routing and expiration constraints (1 yr expiration for most fares).

You can usually change your OW ticket routing for remaining sectors (as long as they stay within the rules after the change) with a change fee of US $75 to US$100 or equivalent. Beware that ticketing, especially if you need a lot of sectors can be a pain and mistakes are prone to occur. Cross-check after every routing change.

I did not know much about RTW until a few years ago and FT was a main source of learning (by lurking here). Also, you can search for "RTW" on internet search engines (Google, etc.) and find some othe useful links. If you are interested I can post some of the links I collected from the various threads on FT.