I have a specialized killstreak Australium sniper rifle with ~3000 kills. I'de like to make it professional, but ideally without losing the kill counter. Am I able to just apply the professional kit without worry?
When you go to "restore" an item, it gives you a list of all things that can be restored/removed. To make it Pro Killstreak, all you have to do it select remove Specailized Killstreak and avoid the button to reset the Strange counter. Then, you just apply the Pro KS kit normaly. As long as you do not trade the item or select the button that resets the Strange counter, the number of kills on the weapon will never reset.
The final answer for the question is yes.