Ehm, usually manufacturer will not go to the extend of voiding the warranty due to custom OS installed by customer , the most they will load back with the default OS when the product is in need of servicing..
The best bet to see whether the product support win7 or not is to go to the manufacturer's website and see whether they have the win 7's drivers available for download or not...if it does, at least u can be assured that it will have the drivers to support. Just need to get a oem/retail version win7 and start downgrading.