After all, you are what characters (not bytes) required for contents? The maximum number of characters that can be stored in VARCHAR is 21845 UTF characters (not bytes). If 20,000 characters is enough, you should change content to VARCHAR(20000). If you need any more, you should use MEDIUMTEXT (LONG VARCHAR).
Basically, even if you have changed field type and field size, failure will not occur. However, surely the performance will be degraded. The extent of degradation of performance will be dependent on your hardware. If high spec CPU is equipped on your server, degradation would be a little.
In addition, if you changed program OR database schema, there is a need to be careful to upgrading core and adding plugins. For example, if you add specific plugin which is necessary more than 8000 characters in content field, that plugin will change content field with "ALTER" query. In other words, the changes you make will be overwritten by the plugin. This change is carried out in a place and timing where it is not visible to your eyes. So, you may not notice that. This is one example, but there is such risk.