What you've already done is a great first step. Here are two additional ways you can improve conversion:

A)
Right now you have two options
- Free
- $ + pro features

To get more people to select option two, you can add a third option. This third option is a 'dummy option', you won't be expecting many or any people to actually choose it. It's just there to make use of a known way in which the human brain makes decisions. The third option should cost more than [option 2], maybe 1.5X the cost. The third option should be [option 2]+anything you can think of that could possibly add any amount of value. For instance, maybe it's access to a special "pro-user FAQ", I don't know exactly what your services are so you'll obviously know better. When this third option is present users will see the middle option [option 2] as a good deal, or at least a better deal than they had previously seen it as. This will lead to higher conversion.

B)
This method is pretty commonly known and used, but maybe you haven't implemented it yet.
Studies have repeatedly shown that the human mind is more inclined to fear loss than to the seek gain.

So make the pro version of all your services available for free for one month. This will accomplish two things.
- Once they have signed up for free, they will have a lower likelihood of canceling than the initial likelihood of them signing up.
- In this process you will be collecting a ton of data on what your users want / don't want.

Only notify existing users about this free offer through email (do not advertise it on the site), and mention in the email that it's because they've already given you something, and they're already committed to you to some extent ("we love how you have been using our products", or "because you've such a great user for such a long time").

To use the pro version users will have to give you credit info, and start billing them if they don't cancel within a month. Make sure that if they cancel you have a question about why they are canceling. It may be because 1) they don't want to pay that much, 2) you're missing things that people want ("I wish you had this feature"), or 3) you have some things that people don't want ("there's a bug when I do this"). If it's #1, then you can start advertising to a new user base, or make your prices cheaper, and if it's #2 or #3 you can have your devs fix things.

_Only_ notify the users that are not currently paid subscribers (if send the email to paid subscribers too they will be pissed that they aren't getting a free month too). Still though, there is a danger that your existing paid customers might find out. So to avoid potentially pissing them off, you can send each of them and email offer to use a service that they are not currently paying for free for a month.

C)
If you want to go more in depth into these methods and additional methods we can talk about it,