Hi,
As you said you have 5yrs data.if you reload the historical data(huge data) to dso without deletion memory issue will arise. so that'y i suggested better delete dso data aslo.
if your DSO all key fig are Overwriten then reloaded whole data will be over written.
For info cube there is no solution. if you want load historical data for the same cube then drop and reload is mandatory.
Thanks