In the worst-case scenario, you'll start with most of your speech being unintelligible, i.e., native speakers don't know what you're saying. This is more like if you're going from, say, English to Chinese than, say, Spanish to Italian.
If your accent does start out unintelligible, it won't stay that way for long. Once you start speaking (and this should be happening early and often), you'll quickly reach the next milestone of most of your speech being intelligible, i.e., native speakers know what you're saying, even though your accent might very well be extremely strong.
Then it's time to get settled in for a bit, because the path between merely being intelligible and sounding like a native is often a long one. Over the course of plenty of speaking, you should be able to push your accent forward. As you get closer to sounding like a native, you'll probably need more concerted efforts (as opposed to merely speaking) to work out the kinks, but doing so will get you closer and closer to the native milestone.
That said, there's not necessarily a need to sound like a native. If you can do everything you need to do after the intelligible milestone but before the native milestone, then you could very will be happy with leaving it at that.