Can I use a provisional patent application I recently filed as part of new, related provisional patent filing?

Yes but be very clear in the patent that it is an improvement over the original patent - note that this could be used in combination with some of your actions to bar your obtaining of a patent on the entire second patents's material. The material in the first provisional that is in common with the second provisional may not be patented if it was disclosed despite the fact that there were provisionals filed for both of them. You cannot just extend the length of a provisional by filing another provisional with minor changes to the first provisional if there has been a disclosure.

You can refer to it but doing so has no legal effect. A provisional application is designed to get you a filing date, providing a regular application is filed within a year of the provisional filing date, and provided that the provisional meets "best mode" and "enablement" requirements of a regular patent application. In my experience, many provisionals are seriously flawed and may not provide a filing date. The patent claims of a patent application define the scope of protection being sought. Any subcombination of disclosed elements can be claimed. To answer your question, if you've made improvements, you can file a new provisional that discloses everything from the previous application and also discloses the new material in connection with additional figures. Then in your regular application, your claims can focus on the new subject matter, the old, or a combination. I strongly recommend that any provisional be drafted by a patent attorney as if it was a regular application to increase the chance that the resulting patent will be valid and infringed. The patent cases are a minefield of traps for patent drafters who do not keep up on the law and do not employ best practices. The provisional will be part of the file history and mistakes made may not be curable. Many applications drafted by professionals are found to be invalid. Amateurs don't stand a chance. Also, be aware that foreign filings will be due in one year from the provisional filing date.