redirect to a specific page, but keep the Querystring?

Yes, the syntax is different in 4.2, the string composition using '+' is not available. In version 4.2 you can use the %u expansion in the redirect string to get the original URI, but there is no way to get just the query variables. It means that the server needs to accept the extra component in the URI. Your rule may look like this:

if (http_host == "partnertraining.host.com") {
    redirect to "https://education.host.com/application/%u"
}

Please, refer to section "Using format strings (expansion characters)"

of the BIG-IP Reference Guide for details (http://tech.f5.com/home/bigip/manuals/bigip4_5_10/bigip4_5_10ref/BIGip_pools.html1184371).

So, if for example the original request is:

http://partnertraining.host.com/login.asp

The redirect will be to:

https://education.host.com/application/login.asp

You'll need to use URL rewritting of some sort (for example mod_rewrite in Apache) at the target server to do the rest. You can always upgrade to a more up to date version of BIG-IP and use string parsing and composition to construct the redirect to URL :-).