[votan]

If working with ZO_DeepTableCopy, the resulting table is not referenced by the saved variable. => not serialized to disk.

The questions are:
1. What is the expected behavior if you switch from char to account? Current settings taken over? Or switching to account settings?
2. What is the expected behavior if you switch from account to char? Current settings taken over? Or switching to char settings what ever they were?

I would try this (untested ):
At addon load (EVENT_ADDON_LOADED):

SimpleXPBar.AWSV = ZO_SavedVars:New("SimpleXPBar_Settings", "1", nil, SimpleXPBar.default_settings)
SimpleXPBar.CharSV = ZO_SavedVars:New("SimpleXPBar_Settings", "1", nil, SimpleXPBar.AWSV)
     
if SimpleXPBar.AWSV.general.account_wide then
    SimpleXPBar.CurSV = SimpleXPBar.AWSV
else
    SimpleXPBar.CurSV = SimpleXPBar.CharSV
end

If account settings are new, defaults are used. Then, if char is new, account settings are taken as default. (Which are default, if account settings were new)

Checkbox:

setFunc = function(val)
    if val then
        -- purge data, due to different version
        ZO_SavedVars:NewAccountWide("SimpleXPBar_Settings", "2", nil, nil)
        -- purge again, back to old version, take over char settings
        SimpleXPBar.AWSV = ZO_SavedVars:NewAccountWide("SimpleXPBar_Settings", "1", nil, SimpleXPBar.CharSV)
        SimpleXPBar.CurSV = SimpleXPBar.AWSV
    else
        -- purge data, due to different version
        ZO_SavedVars:New("SimpleXPBar_Settings", "2", nil, nil)
        -- purge again, back to old version, take over account settings
        SimpleXPBar.CharSV = ZO_SavedVars:New("SimpleXPBar_Settings", "1", nil, SimpleXPBar.AWSV)
        SimpleXPBar.CurSV = SimpleXPBar.CharSV
    end
    SimpleXPBar.AWSV.general.account_wide = val
    SimpleXPBar:UpdateStats()
    SimpleXPBar:UpdateValues()
end,

Alternative, you could access the saved variable more directly.