local function upgrade(from_entity, to_name)
    if not blacklisted(from_entity.name) and from_entity.surface.create_entity{ name = to_name, position = from_entity.position, force = from_entity.force } then
        from_entity.destroy()
    end
end


to something like


local function upgrade(from_entity, to_name)
    if  entity.type == "radar"
		and entity.name ~= "radar"
		and ( entity.name == "rt-upgrade-tech-1"
		      or entity.name == "rt-upgrade-tech-2"
			  or entity.name == "rt-upgrade-tech-3"
			  or entity.name == "rt-upgrade-tech-4"
			  or entity.name == "rt-upgrade-tech-5"
			  or entity.name == "rt-upgrade-tech-6"
			  or entity.name == "rt-upgrade-tech-7")
		and from_entity.surface.create_entity{ name = to_name, position = from_entity.position, force = from_entity.force } then
        from_entity.destroy()
    end
end