Can people improve their memory by training themselves to recall previously forgotten memories? Apparently, somebody has recalled previously forgotton memories and become able to speak fluent french after a coma. I'm wondering if people can also train themselves to be good at recalling previously forgotten memories by training themselves to recall information they've forgotten that there's a record of.

For example, if a cell phone gets invented where you can type in somebody's name and then type what their phone number was and when you want to phone them, you have to go to their name and then start typing digits from beginning to end from memory and as soon as you type a digit incorrectly, you get notified and then you have to type that digit followed by the next digit and will be notified when you do so correctly, I wonder if the people using it would train themselves to recall memories of what phone numbers were.

I wonder if people would become even smarter if some of the memories they trained themselves to recall were memories from before they started the training because that way, their brain has to form complex connections in order to recall memories that were lost in the brain than to just not forget stuff they learn from then on. That's probably because lost memories get scattered to unrecognizability in the brain but that doesn't mean there isn't a complex method of extracting the memory from the disorder. Maybe unlike the man in the previously linked video who thought he was somebody else, with sufficient training, they'll learn how to properly extract their past experience correctly from the disorder and have undistorted memories. Maybe getting an actual defect in the connection suppressing ability making you do something like only have suppress unimportant connections from an accident like in that video could make it so easy to recall previously forgotten memories cause of not suppressing the connections for extracting them that much and be superior to any possible training to recall previously forgotten memories because you'll be hyper-aware thinking about a large amount of your past all at once because it's just as easy as thinking about a single past experience because the memories of the individual experiences were already lost in the brain, and the you can recall memories that were even more badly lost with the defect in the connection suppressing ability than you would with training alone.

I wonder if in general, it's easier for autistic people to train themselves to recall previously forgotten memories.
Memory is not fixed - it is very plastic.  If you look at Amnesia, there are many different causes and many different ways it can manifest itself. It can also be anywhere between mild and severe.  There are methods which can be used to try and prevent further memory loss - by abstaining from alcohol when it is caused by alcohol abuse such as with Korsakoff's syndrome, by using diaries, calendars, wall planners etc. But just like with Alzheimer's Disease there is no effective (and ethical treatment) for memory loss.

Which brings me onto the ethics of memory recall therapy or training.  If you look at studies into memory and PTSD (Post Traumatic Stress Disorder) - also found in category F43.1 in ICD-10 - methods used to try and help sufferers to remember the traumatic events for this example, have been found through research to cause False Memory Syndrome.  Although as with many pieces of research, there are opponents to the idea.

From what I know, and believe from looking at research, once memory is lost, it is lost for good.  You may recall parts of the memory through various ways but to try and recall the whole memory can lead to a change in what actually happened as @Arnon pointed out in his comment.