@RainbowCrayons
I have read that they would be able to tell if it had been altered artificially.
You can identify alterations made using some old techniques but there are other techniques (e.g. serial passage and 'no-see-em' approaches) that do not leave the hallmarks of alterations. A bit like saying you can tell if someone has had their appendix out due to scarring - well, using old techniques yes, but using modern keyhole surgery, maybe not. Not a perfect analogy but you get the idea.
OP
The Wuhan Institute of Virology (WIV) did famously study bat coronaviruses. The WIV was known to carry out Gain of Function experiments. These are experiments where you change the virus to improve transmissibility or introduce the ability to infect new species. The idea is to study how viruses may evolve and learn how to combat them ahead of potential pandemics. The obvious concern is if the altered virus escapes.
It's also true that while WIV had BSL-4 labs (highest safety level) work on novel coronaviruses can be carried out at lower safety levels. In addition, all it takes is one person messing up once, or one piece of equipment failing once for a virus to escape.
It doesn't make logical sense to me that covid-19 was engineered as a bioweapon and released deliberately. What gain could there be from that? Especially with it starting in Wuhan. However I can well believe that a natural virus or an altered natural virus that was being studied got out accidentally. Lab leaks happen all the time, including a fatal smallpox leak in the UK. SARS (the original one) leaked from labs in both China and abroad multiple times - sometimes causing deaths.
However I will also note that the reason bat coronaviruses were being studied was because they has already caused one natural epidemic and there was a strong possibility that there would be more. It is entirely feasible (and has for many months now been the scientific consensus) that the pandemic was caused by natural evolution of a virus in the community i.e. not in a lab.
Anyway, to answer your main question, whether or not it was natural or altered, viruses like this one mutate constantly. Sometimes the mutations are to the benefit of the virus, sometimes to its detriment. Mutations are accidents when the virus replicates itself not planned changes. It's just that if the mutation is a hindrance to the virus that strain will probably not infect as many people and will not become dominant.