"My struggle here is what should be BRD be like in case we have to install this new application no matter what before the expire of application support even though we don't know what are the new features and functionality of the new version and change in the process"

Based on Knowing Nothing about the new product ... the Business Requirements to me would basically be that the new system, at a minimum, perform like the current system you are using ... with minimum training. You can always write stuff about some screen needs to do this or that but you are buying a packaged product where the stuff is already done ... unless you contract with them to make screen / system changes to fit your company needs as needed ... or have the contract to allow you to have the source code and make your own changes ... we have done that with some COTS products and that company allowed us to do changes BUT if they liked the changes we did, they could use that code in their future releases.

Many times, software companies have a Demo system you can get to install and review ... it might be good for 30 days and then it stops working ... at least you can install it, review it and see if still fits the company's needs. Check out all the screens and reports and see what is good, bad, difference ... see if there are any gaps between what you have and what the new system offers ... get a Stakeholder to review it with you and see if they like it.

See if they have some local sales office that you might demo the new product ... if not, then maybe you a Stakeholder, technical person can fly to their location and demo it there ... asking all types of questions ... that you prepared before you went. Sometimes we got names of local companies that are using the product and made an appointment with them to come and see it ... and get their feedback. 

Check out the database and see what has changed ... see how you will migrate the data from the old system to the new one (data mapping) ... maybe they supply a tool to do that.

If you do get a demo and it has been changed in ways that don't support what your company needs, it might be time to look for another product before you invest money in that new upgrade. 

Hey ... Have Fun.