My ac system is 240V. I want to step it up to higher voltage to plug in some temporary equipment that needs at least 400-440 volts. If I use a 120v-240v step up transformer.. would it make my 240V become 480V?
Yes, for a short time, and then there will be a BANG and a fire will start.
Whether that happens in 5 minutes or 5 days is based on the quality of the build of the equipment. Siemens >> random junk off Alibaba.
If you plan to do that, then buy a 240/480 autotransformer in the first place, which can also be used for 120/240 at half the VA. This type is more likely to be available as an isolation transformer, for what that's worth.