Musk stated the current battery packs have a lifetime of 300-400K miles and that new battery packs next year will have a 1M mile lifetime.
When Tesla started selling Model S, they briefly considered offering a battery replacement plan, allowing owners to pre-pay for a battery replacement in the future. They quickly dropped the program.
Tesla also supported battery swapping (which may have been done to get some incentive funding) - and that would have eventually created the potential to swap for a pack larger than the owned pack, with the ability to purchase the upgraded pack for the difference in battery value. But Tesla discontinued the battery swapping program.
And since then, there hasn't been any mention of battery swaps or replacements. The only range increases have been with software limited battery packs, when the owner may be able to unlock the extra battery capacity after purchase.
With a 300-400K mile lifetime, Tesla doesn't have much incentive to introduce a battery replacement or upgrade program. Plus, as Tesla introduces technology improvements, there's no guarantee the newer packs will be backwards compatible with the older vehicles.
For anyone planning to purchase a new Tesla (or any EV), you should not count on ever getting a larger battery pack. The rated range at purchase will likely be the maximum you'll ever see, with some degradation over time.
At some point, 3rd parties could get into this business - especially for the much higher volume 3/Y/pickup vehicles, offering to refresh battery packs to restore them to close to the original range - or do an upgrade to long range.
Instead of planning for battery upgrades/replacements, Tesla seems to focusing more on recycling old battery packs and recovering as much of the raw materials as possible, basically allowing them to take old batteries, grind them down and then use those materials to build newer generation battery packs.