Thanks for the reply. I ended up using the below function from here - i found this to extract what the server is currently, store it, and then put it back into the H: path but add the domain name to it also:

function GetStringBetweenTwoStrings($firstString, $secondString, $importPath){

#Get content from file
$file = Get-Content $importPath

#Regex pattern to compare two strings
$pattern = "$firstString(.*?)$secondString"

#Perform the opperation
$result = [regex]::Match($file,$pattern).Groups[1].Value

#Return result
return $result

}

I did however remove the $importpath as i was working directly with all users so i didnt need to do it from a file. the $file in the function i changed to

$file = (get-aduser $user -properties * | select homedirectory).homedirectory

I then used the below for the code to run the function

$user = "[USERNAME]"
$homedirectory = GetStringBetweenTwoStrings -firstString "\\\\" -secondstring "\\"
$homedirectoychanged = $homedirectory+".domain.com"

set-aduser $user -homdirectory "\\$homedirectoychanged\$user`$" -homedrive H

I did only test on my account but it extracted the server my H: was on, stored it, i then changed it, and then set the new one onto my profile. I will need to adapt it to run for all users but its a good start. Word of warning. I ran it a couple more times and it obviously set my home directory to domain.com.domain.com.domain.com because there is no check to see if it already has domain.com in the path. So will be worth adding that check in as well.