downgrade from Android 4.xx to 2.xx can we down grade Android from 4.x to 2.x?? if I bought Android 4 device and need to downgrad it to Android 2.xx, can I do that ?

the devices are HTC with these modules:

•HTC devices especially the Incredible 

•HTC Wildfire running CyanogenMod 

•HTC Desire 

•HTC Wildfire running CyanogenMod 6.1.0 

but when I buy them they will have Android 4.X
It depends on the device, and what you mean by "2.xx". Do you want A: stock Android? B: a ROM? C: a manufacturer-skinned stock image?

I'm going to assume that the answer do the question above is D: Any of the above. I'm also going to assume that by "2.xx", you mean 2.3. There are a few general requirements that will need to be fulfilled for it to be practical, and remember that this depends on the device and software; do your research before buying (assuming you haven't bought already).

The requirements:

1: The device was launched with Android 2.3 or earlier.

2: The device has an unlocked, unencrypted (or unlockable) bootloader.

3: (Pseudo-requirement) The device had, at the time that Android 2.3 was still important, an active development community.

If you meet the requirements:

You will have to do some google searching for various solutions. If you want...

A stock device image (not stock/"pure" Android) then you can find instructions generally by searching "unroot [device name]" on Google and XDA Developers.

A ROM: Go to XDA Developers or the popular dev community site for the device (Google is your friend) and search for CyanogenMod 7, or any other 2.3 ROM.

Stock Android: Good luck. I don't think you'll be able to find any flashable images with stock Android 2.3 for non-nexus devices.