If you solder it well, there won't be any measurable damage on the inside. Lithium ion will fail under heat stress through:
- Ion reduction (long-term heat, arrhenius effect, +10C = 2x faster ageing)
- Thermal runaway (140-170C)
- Thermal dissociation of e.g. the electrolyte
When soldering the cathode, which is connected both electrically and thermally to the entire outside of the battery, it's extremely hard to introduce enough heat into the battery to cause any of the failure modes. You'd have to be heating it for minutes to get anywhere near dissociation or runaway, and even then you don't have any current through the battery to kickstart the runaway reaction. Mind you, lithium ion doesn't use water-based electrolytes. There is no pressure build-up at any time, unless if you're producing hydrogen during thermal runaway.
On the anode, you're generally not directly thermally connected to anything but the carbon/polymer rod, so 1) you'll be done in seconds, way too little time to introduce any significant amount of heat into the pack and 2) you have very bad thermal contact to the sensitive bits, so I doubt if you can even get the electrolyte past 100C
Spot welding is not necessarily better than soldering. The reason cells are spot welded is simply because it's economical, however you don't get a particularly low resistance weld. The actual welded area is only a few mm2, good enough for laptop battery packs that never go above 1A discharge, but not nearly enough for high-current applications like RC stuff. Soldering is best practice in this case, and if you half know what you're doing it's not going to impact longevity at all for any practical purposes.
Hell, up to a few years ago most high-end electric tool manufacturers used special high-copper solder to build their packs (instead of spot welding). Now that surface welding has matured they've moved over to that, but that should say a lot.