Converting your old model heater to best tankless water heater is simple. But still need professional to do this job. You can not take it by yourself except you are an expert in this field. Budget needed to do this job is about $300 to $800. It is different based on what service you want and also supplies.
First things to do is read the tankless water heater reviews, this can help you to decide which one is the ideal products for your needs. You can get recommendation along side with products reviews and price that you can consider or think about.
Then you can buy it online or at local stores near you. After that you can find the local service provider or contractors to install it on your house and remove any installation of old model that doesn't needed anymore.
To get it right, you can check the best tankless water heater reviews page and use it as your guide.