So, I switched from Lich to Gold Dragon, and I lost my undead companions including my Skeletal chamption - and this is totally fine.
However, the Lich spells were still with me after switch, which also seemed logical, because otherwise you lose all benefits of Mythic levels 1-7, since Golden Dragon starts on Mythic level 8.
But the first problem with Gold Dragon started here - why Dragon’s mythic level is not added now to my caster level? This ruined completely my Sorcerer’s build (and will ruin every caster-based class, because they even lose the 9th level spells, not speaking about spells DC, spell penetration and so on). This is illogical even lore-wise - dragons are always powerful spellcasters, even the most known prestige class for Sorcerers is “Dragon’s Disciple”. Why a real dragon doesn’t count for my caster level?
The second problem revealed itself after I decided to retrain my min character, so it is more tailored to the new mythic path. And all Lich-related abilities, which existed before for my character, disappered completely, including all Lich spells!
So, as you can see, the character not only lost all Lich spells, but the number of spells learnt is also ridiculous in comparison to what it had before (due to absence of merging of Lich and Sorcerer spell books).
The Gold Dragon looks hardly playable because of these problems, for caster classes at least, especially on difficulty higher than “Normal” (I used to play on “Core”, and finished the game being pure Lich already, this time wanted to also try Gold Dragon, which doesn’t look playable at all).
There is another thread, where other common Golden Dragon-related problems described in detail.