The answer, as in many areas of genealogy, is "it depends." Because different people test with different companies, many of which maintain their own databases of tested individuals, you will achieve the greatest chance of useful matches by either being tested, or sharing your DNA results, with as many companies as possible. The big three used by the vast majority of genealogists are AncestryDNA, Family Tree DNA, and 23andme. Geno , sold by National Geographic, is also popular, but it tests purely for ethnic heritage (deep ancestry) and is not useful for learning about possible ancestors during a reasonable genealogical timeframe.
Research your new career field thoroughly and present a detailed explanation about why you are making the change. Speak with contacts working in your new field and shadow them to gain a more in-depth view of their work role. If possible, intern or volunteer for a short time to show prospective employers that you are knowledgeable about the field and committed to your new role.
If you think your employment history might raise a red flag in regards to your level of commitment to each job you've held, emphasize how your past experience has led you to this field and the ways in which is bolstered your confidence for this new position.
For example, you might mention that in the past, you changed fields because of the allure of a higher salary, more responsibility or greater status, but didn't carefully consider how stimulating the actual work would be. You can mention that you grew from that experience and are ready to fully commit to a new position.