This is a tricky question to answer because GETs might be totally fine, or might be totally broken, it depends on how you've built the security mechanisms on your API. Have a look at the OWASP REST Security Cheatsheet.
In some cases there's no difference between URL parameters in a GET, and body params in a POST. For example, take RFC 6960, the wire protocol for Online Certificate Status Requests (OCSP), appendix A.1 allows the same data to be sent in a POST or a GET (if under 255 bytes), and must be treated the same by the server.
HTTP-based OCSP requests can use either the GET or the POST method to
submit their requests. To enable HTTP caching, small requests (that
after encoding are less than 255 bytes) MAY be submitted using GET.
If HTTP caching is not important or if the request is greater than
255 bytes, the request SHOULD be submitted using POST.
In general though, I think POSTs give you more control over security. For example, many phishing attacks work by getting the user to click on a link. All browsers treat link referrals as GETs, so if a particular resource is only accessible over POST, then it's not possible to do link-based phishing attacks (or, at least, the attack requires multiple steps, like getting the user to click the link and then click a button on the phishing page).
Another win for clean separation between GETs (used only for fetching data in a "Read" type way), and POSTs (used only for modifying or storing data in a "Write" type way) is cross-site request forgery protection [source]:
In HTTP GET the CSRF exploitation is trivial. For example, a simple hyperlink containing manipulated parameters and automatically loaded by a IMG tag. By the HTTP specification however, GET should be used as a safe method, that is, not significantly changing user’s state in the application. Applications using GET for such operations should be rewritten to use HTTP POST and/or use anti-CSRF protection.
TL;DR1: It's hard to say that switching from a GET to a POST will magically give you more security, because it depends what that API endpoint is doing. POSTs do give more options for how you implement your security mechanisms and a few less attacks to worry about, but with the right understanding and attention to detail you can build perfectly secure GET APIs.
TL;DR2: It's hard to answer your question by itself, out of context of what your API is doing, and what security mechanisms you have in place (CSRF protection? How does you authentication and authorization mechanism work? etc). I would suggest that you read the OWASP REST Security Cheat Sheet and look at your API security as a whole.