I would also imagine that the shares would be requited in the new currency, just as it was when the switch came to the Euro.
While a rise in value of a new German currency would hurt exports in the short term this would not be a long term effect. Imports would equally fall and with Germany importing all oil, pretty much all gas, and now becoming an increasingly large electricity importer this would have a huge positive effect on the production cost of German exporters who would then be able to reduce their prices.
Remember, since the birth of the GDR, the Deutsche Mark was always one of the strongest currencies in the world, and it was because of this that they had such an economic boom. In other words, strong currency means cheap imports (especially raw materials) and lots of capital attraction, the very things that would help German exporters.