Why could changing my custom field names cause problems?
Changing the names of your custom fields may cause inconsistencies in your email messages because they are stored by number, not by name.
What Does This Mean?
For example, let's say you have a custom field named "Hair Color" you collect from your subscribers in a sign up form. When you created this field, it was assigned a custom field number, somewhere between 1 and 25 - the maximum number of custom fields you can create and use.
When a subscriber fills out your sign up form, the information is stored appropriately in whatever slot number the field was created in, and if you look at that's subscribers information, whatever they entered for "Hair Color" is found in the "Hair Color" field.
Now, if you decide to change the field name, you can, but only the name changes. The information stored in that slot or number for that field remains the same. For instance, if you change the field name to "Shoe Size" to collect in your form, subscribers who previously entered their hair color will have whatever they originally entered now in the "Shoe Size" field.
In other words, some subscribers will have "Hair Color" stored in the field while others will have "Shoe Size" information stored.
How Does This Cause Problems?
If you're using this custom field in your messages, or if you're using this information to segment subscribers (e.g. if you want to send a message to subscribers who have a certain shoe size) you may see inconsistent or unexpected results. In our example, when using {!custom Shoe Size} in our messages, subscribers who entered their hair color into this field (when its name was "Hair Color") will see their hair color where they would expect to see their shoe size.
What Can I Do To Avoid Problems?
Just take a few additional steps when renaming your custom fields. If you already have subscribers and information stored in a custom field, review any messages where you might have used the custom field information previously in case you need to make changes to them.
If you're just looking to add a new field, it probably isn't necessary to replace an existing one. Just leave your old fields in tact and add a new one.