Also worth adding Gundam, Macross, Kinnikuman and Saint Seiya (when including spinoffs).
Like all things, execution is key to this. If done well, such as in JoJo's Bizarre Adventure and Gundam, it's a good way of adding to the scope of the world and justifying the continuation of a franchise (though I wouldn't say this is the case for Gundam's alternative universes which only have Gundam in name and not spirit). For instance, JoJo's main cast are usually completely different in personalities from that of the previous part and each part has their own unique setting and identity (other than Part 8 which is just millennial Part 4) and Gundam's many series of the Universal Century offer quite a good space opera when viewed as a whole (so long as you treat CCA as the cutoff point). In some cases, it can also offer a sense of realism as a story doesn't stop with the death of a certain character. Whilst not an actual franchise and an adaption of a historical epic, I will include Sangokushi as an example with all of it's initial cast having died around 75% of the way through the series' run, yet what they left unfinished in the war of the Three Kingdoms provides enough of a reason to continue it.
Some sequels can also be a good revitalisation. Whilst something such as Boruto is a clear cashgrab due to not learning from the failures of the original series, a next generation sequel can sometimes do justice. For instance, Kinnikuman Nisei did a good job compared to the original by adding a new cast with some clear differences to their predecessors whilst making the ageing cast of the first series far more complex such as Brocken Jr., Sunshine and Ashuraman.
If done poorly, however, it just exists to slap the name of an established franchise onto a product and make money. Yu-Gi-Oh! is very evident of this as after the original manga's run, it was pretty obvious it had become a means for the series to sell new cards rather than improve the story. Same with Pokemon's games and manga when new mechanics constantly get introduced to retcon older events and it took until ORAS to offer any sort of in-universe explanation, as well as Macross' series after the original just existing to sell CDs. Whilst some Saint Seiya spinoffs can be alright such as Episode G adding more focus to the Gold Saints and helping offer some further characterisation to them, the other spinoffs such as Omega, Soul of Gold, Saintia Sho and especially Episode G: Assassin are just poor attempts to cash in on fanservice through new Cloth designs and fanfic-esque battles.
Tl;dr Writing and the necessity of extension in-series are the key to deciding whether or not changing the cast completely is a good idea.
