There seems to be some confusion on how to apply the said regulation (§308 Nr. 5 BGB) in this case. The given article is a part of the Recht der Allgmeinen Geschäftsbedingungen - generally abbreviated AGB - (roughly translating to "law of general terms and conditions") and to answer the question it is necessary to understand its meaning and context. So it's worth giving a quick overview of what that actually means.
Every pre-formulated contractual obligation, especially general terms and conditions, which is not subject to individual contract negotioation, is subject to the AGB law. AGB law imposes strict limitations on these terms to mitigate power imbalances between large corporations and consumers. These limitations are quite extensive and it usually requires a specialst lawyer to carefully craft AGB that would stand the test of a court.
Creating valid AGB has two aspects which have to be clearly separated:
- The terms and conditions need to become a part of the contract. There are quite a few things you can do wrong with that and if you do, your AGB will not be part of the contract and thus they won't apply at all. This aspect is regulated in §§ 305-305b BGB.
- Even when the terms and conditions become a part of the contract, there are strict regulations regarding their content (Inhaltskontrolle der AGB). This is in general the most tricky part which requires experts to do right. The regulation you refer to, § 308 Nr. 5 BGB belongs to that area.
Having said that, what you do is to apply a regulation of the second part to an issue regarding the first part, namely how changes to the ToS can be communicated in a way that they are valid. It basically means that you are using § 308 Nr. 5 BGB wrong.
A quick information what § 308 Nr. 5 BGB is actually meant for: According to German law silence can never be a legally binding statement. If a contract is proposed to you and you don't make any statement, there will be no valid contract, because you have not agreed to it. § 308 Nr. 5 BGB forbids overriding this rule in a pre-formulated contract, except in a few specific cases.
You however want to do changes to your AGB after they have been accepted by your customers. To make these kind of retrospect changes is a difficult issue by itself and - unless you have provided court-proof regulations in your AGB to allow that - might not be even possible at all. And even if it is possible, you must communicate all changes to your customers, even if they are mere spelling corrections. If you don't, the old reulations will continue to apply.
Whether your changes will actually have an effect on your contract can not be evaluated unless you provide the specific contractual clause. Spelling mistakes that don't create ambuguity or give the clause a new meaning might not need correction. But you should be aware that sometimes even little differences in a contract can make the difference between winning and and losing a disputed case, especially when they are subject to AGB regulations.
The best tip I can give you is to seek advice by a lawyer specializing in AGB law to evaluate the changes in your contract and whether you are able to apply them to your existing contracts.