If the existing investors asked for the 10% option pool when they funded you as part of the terms of a financing, it is not typical to give the equity back to the founders. They may also have protective provisions against this.
You are right philosophically--we advise our startups to have as low an option pool as you can because you can always expand it later and then everyone--including the investors--will pay for it.
When you take the shares back yourself, you're effectively making your current investors pay more for the future option pool if/when the board or a future investor requires you to expand the pool. It is very likely your Series A investor will ask for a minimum of a 10% unallocated option pool--as that is the market.
If you have the right to take back the shares, you can. It won't prevent the future investor from funding, they will just require a 10% option pool or more as part of that--and at that time when the new options are issued, everyone will dilute to pay for that--including your seed/angel/existing investors. So this move is basically imposing greater dilution on your existing investors. And you have to decide if that's worth it.
If they asked for the 10% option pool in the term sheet, it's not typical for an investor to give that back. The issue for them is that if they give up the shares to you, and then the company has to re-issue employee options later, they will have to dilute by that expansion amount.
Having said that, usually the argument for re-upping founder shares has to be made from the posture of retaining the existing founding team. So if you can make that claim without causing alarm it might be one path. You can demonstrate how the founding team is taking on roles you normally reserved for new hires that you no longer need. And to retain them you want to allocate these shares to them. Most investors will still not like this, but it's one possible approach.
The next investor can also ask for a carve-out to re-up the team if they feel the founders are too low. The next investor will care about the founders' equity positions, but I imagine your current investors knew the current cap table and were fine with it when they asked for the option pool carve out.
I think another path can be to reissue the options to yourself and strike an agreement where if the option pool is expanded again, you will allow the existing investors to buy your founder equity at that round's price to compensate for any dilution they may have incurred because of the re-issuance of option pool shares. If you take back say 5% of the option pool and give to founders now, and then re-issue 5% later, your investors will lose around 2% points of dilution, and you could offer them to buy 2% of your shares late -- which you may be fine w/ anyways to get liquidity as part of a future priced round and mitigate their dilution concerns.
The best bet is to get a new VC to clean up the cap table and issue more shares to the founders and work with them to make this part of their term sheet.