No, you cannot "just" change the license to GPLv3.
The MPL2.0 is a per-file strong copyleft license, with an opt-out provision to incorporate MPL-licensed files into a larger project that is under the GPL license. This means that if you only made changes to files containing MPL-licensed code, then you cannot change the license.
Also, if the author of the MPL code oped out of the GPL compatibility (by adding the phrase "Incompatible With Secondary Licenses." to the license notice), then you cannot change the license or use it in a GPL-licensed project.
To be able to publish your app under the GPLv3 license, your app must contain at least one file that does not contain any MPL-licensed code and that is already or can be licensed by you under the GPLv3 license. That triggers the compatibility clause in the MPL license and causes the MPL-licensed code to become dual-licensed under both the MPL and the GPL.