If you don't import the old comments, they will still be there in the old comments field. You can recover them later.
There is one point where I'm worried you might lose data, though. Suppose a contact is in both Eloqua and Salesforce, and they already have a comment in the comments field. So you create a new comments field (which is empty) and map it to the Salesforce comments field. Then this contact gets published to Salesforce for whatever reason (they become a hot lead). So now the previous comment, which was in the comments field in Salesforce, gets overwritten with blank. The previous comment is still in the old comments field in Eloqua, but it's no longer in Salesforce. To a Salesforce user, it will look like the comment disappeared for no reason.
Other than that, yes, you could just create the new field.