Iterating

The natural progression is that you will make big changes to the game at first, and the changes will generally get smaller over time. When the changes peter out to nothing on their own, the game is done.

If you've recently made medium-level changes, your game isn't finished.

Change one thing

Just fix the biggest problem, each revision. (You can also make little improvements to tiny things. That's not a problem.)

You want to get feedback from play and from playtesters constantly. Doing a whole lot of changes at once will mean you don't understand which changes led to which outcomes. 

Sometimes, underwhelming things will stay in your game for quite a while. Just change the most important thing, and tolerate the rest in the meantime.

You don't have to fix everything at once, anyway. Some nagging problems will fix themselves, or be obsoleted by larger changes to the game. Just leave them there, and be aware of them.

It's easy to be a perfectionist. It's hard to consciously leave things imperfect.

Don't bother trying to perfect any revision, as though it's the last one.

No revision is ever the last one.

Keep content under control

Medium and large changes will throw off your balance, and make your content not work properly. Don't go and update all your content, beyond what's absolutely necessary. Just remove most of it. You want to test those changes in isolation, and more changes are probably coming. You don't want to have to update all your content each time.

Content creep is common, in board game design. You only need minimal content, to test your game. Your content also doesn't need to be well thought-out at this stage.

Your file should have a large section of "benched" content. It's not dead, just resting. You haven't lost it. When the game is stable again, or you need content, just move it back into the active area.

Testing new things

When you add new things, make them intentionally overpowered. I start things off cheap and gratuitously overpowered.

Being overpowered means that the players will immediately use them, and they won't equate being underpowered with being unfun. Later, I tone down the power.

When putting something new into the game, don't flesh it out. Just put enough in to try it. Build it out later. Also, don't worry about its consequences or interactions. Just arbitrate them on the fly. Do the least amount of work to get it into the game.

Large changes

Your game has parts, and they're arranged into a structure.

Your structure is the core. It is your game.

If you add or remove a major part of the game, you're really designing a new game.

This change ripples so much that you effectively go back to the beginning of design, because everything else has to change.

You can change a core system to something else, as long as it still has the same inputs and outputs. Then, it won't ripple and blow your game apart.

In a recent prototype, the core action selection system changed a few times. One version had a track with tokens that moved, and became available or unavailable over time. Another had cards you could take, to choose your action, that would be returned at the end of the round. A later system involved covering up spaces on a board, and it later changed so that those spaces would never be uncovered again during that game. All of these systems were different, but they always resulted in the player choosing an action for their turn.

Redesigns

Most times I've made fundamental changes to a game have been erroneous.

Many of the people I see endlessly working on the same game have, in reality, been working on a series of related games, and constantly restarting their design.

Generally, don't make fundamental changes to your games. Pursue the game as it is, with some modifications, or abandon it. Go and do at least one other game (ideally several). If and when you return to the game, intentionally start again, with the complete freedom of a new game. Even then, I don't recommend this.

Often, I have a great idea, but I can't get it to work. I abandon the game. The idea rattles around in my head for a long time, and I eventually come back to it. My personal experience of redesigns has been very poor. It might be different with you, but I strongly advise against redesigning. A good idea doesn't necessarily become a good game, and the redesigns often fall into the same pitfalls as the original. There is often just some fundamental flaw with the idea. Just come up with a new idea.