The first step I would say is to do industry/competitor research. Who's already doing a conference for a career transition? I would study their information and reach out for additional details by asking for a 20-minute interview. Most people scoff at this idea, but I've done this multiple times as both the interviewer and the interviewee. There are business owners that genuinely like to help and think there's enough for everyone. This will give you great insight into the challenges, wins, and the viability of this idea.
Second, identify your target audience. Who would be your ideal attendee at this conference? I'm thinking of life stage, industry, background. Who is that person? Detail as much as you can about the person you feel would benefit from the program you have in mind.
Third, shop your idea around your target audience to get input. You want them to help you better understand their challenges and needs which will help you create the right program content and marketing strategy for the conference. (i.e., what are your 3 biggest frustrations in career transition, have you ever paid for a conference like this before? If so, what did you pay for this conference? What did you like best? etc.)
If I can help further please reach out. Good luck!