There are many creative ways of using what you have to increase your SaaS revenue without acquiring any new users. You may think this is hard to do, but it can actually be pretty simple for most SaaS businesses. It just requires you to adjust your thinking away from seeing users as the only dial that can turn into the only dial you can't.
1. Charge More
The most obvious, and usually the most effective. If you're worried about what your existing customers will think, you can use new pricing models to upsell. For example, you can offer additional users at an additional cost. Or start charging for usage, or some kind of "credits" system for buying additional usage past a certain maximum amount. This won't affect most of your low-usage users. Your users who are high-usage are more likely to be affected, but they are also getting more value out of your SaaS and are usually fine paying a little extra for their extra usage.
2. Add On Features
If you're about to launch any new features, considering launching them as paid add-ons, instead of just attaching them to your existing plans. Alternatively, if you know your new feature is going to be popular, you can put it in your highest tier. You will likely see a few users upgrade after you announce the feature.
3. Send Emails
It's possible many of your users don't know what the advantages are of your higher tiers. Send out an email to your low-tier users letting them know the benefits of upgrading. You can even offer them coupons if it's not a lot of overhead for you.
4. Remove Your Free Plan
It doesn't have to be forever. But you might have some users on your free plan who are actually fine with paying if there's no alternative. Or if you don't want to shut it down entirely, see if you can remove some important features in the free plan to make it less valuable to stay with or more temporary.
5. No Unlimited Plans
The same is true of your unlimited plans. You can always get rid of these and turn them into usage-based plans, or split them into even higher tiers.
You can probably come up with all kinds of other ways of rearranging your pricing to incentivize your existing users to pay more. Just get creative, and pay attention to the features your users get the most value out of.