All information regarding pricing can be found on our
pricing page. If you purchase the $99 standard license, yes you can upgrade to the developer license at a later time paying only the difference between the two license prices.
AppThemes does not offer installation or customization services, however we do have a
certified partners page where you can find developers for hire who are experienced at working with our themes. If you make contact directly with an outline of your requirements, they can provide a quote for the work. Thanks.