If KWA has the parts you need in stock you should be fine. However, if you opt for real steel parts then you need to consider several things. Keeping the original tower sight might cause some issues with fit. Before you attempt to make the switch, be sure you take very accurate measurements to insure that the RIS will fit. The KWA specs for the hand-guards on some of the rifles are about 1/16 of an inch short of some manufacturers Mil Spec hand guards. I know this because I changed my front end on my KM4A1 to a 2 pc Mil Spec foregrip and in order for it to work I needed to change the front sight to one that clamped on the outer barrel. Fortunately it was the look I was going for. By doing this I was able to adjust the sight to fit and that required moving it forward by 1/16".
To test this further I attempted to put my hand guards on the my son's KM4RIS and found that the RIS's hand guards were short by 1/16" so my real steel parts would not go on the rifle unless I moved the sight forward.
Hope this helps.