That's a great question - and one I've struggled w/ a couple of times now - we've moved from our primary home in the SF area - rented out a condo that was underwater and didn't cash flow, but covered (like in your case) the mortgage (P&I), HOA, and taxes but didn't cover vacancies, or capEx etc. That was 6 years ago and w/ the help of a PM that screened our tenant - we've had the same great tenant since that first placement so we never had to worry about vacancies. Also given the age of our condo, we haven't had any capEx expense, so basically we've had a tenant pay down our mortgage, and since then the market has rebounded and we are now sitting on a nice chunk of equity.
We're about to come into this situation again due to a military move and I'm contemplating the whole situation all over again. So here's what I'd list out as the pros and cons to weigh (in addition to what folks have already brought up)...
Pros to keeping it as a rental:
1.) Montebello - and most of the SGV is a very hot market - you're seeing some amazing appreciation in that whole area where even the neighboring cities that used to be undesirable have starting list prices of 500K! So I think you do have appreciation on your side.
2.) since this was a primary residence you probably have a better interest rate on it than you would if you were to purchase an investment property w/ the proceeds... and if not, have you considered refinancing it (before you move?) What we've stumbled on is that if we buy a primary, live in it... then have to move, you get to keep that primary residence perk of lower down and lower interest rate and you get to re-use those perks when you move into your next primary home.
Cons:
1.) it's not cash flowing - so that's an obvious con - but can you change that w/ a refi? Can you value add - what kind of zoning is the house in? is there any way to add a garage unit or something like that to get an additional renter in there?
2.) This might cost you money if you get bad tenants - but again this could be mitigated to some degree.
3.) Not really a con, but - a pro to selling is that you can reap the benefits of up to 500K in profit w/o being taxed on it and you won't have to worry about doing 1031 exchange or anything like that... but i think - and others will have to chime in on this one... that you can still rent it out for a couple of years and decide later to sell as long as you had lived in the property 2 of the 5 prior years!
As far as your HELOC goes - you'll also need to factor in how much you're renting out the other sides of your planned duplex and how much of a 'cushion' you will have with that. I'm assuming you and your wife have w-2 jobs... in our case that's helped up to hang on to that original condo that didn't cash flow since we weren't relying on the cash flow to make ends meet. So depending on the condition of your home - and what you can forsee for cap ex expenses that may arise and what your debt to income ratio is right now - it may be advantageous to hang on to that property. While it's true you probably wouldn't have purchased it as a rental from the outset, due to the numbers, you have other positives that you gain from it being a primary converted to a rental.
Best of luck to you! Like I said earlier - we're back in that same position now w/ a house in WA and an upcoming move this summer... so we have a couple more months to kick around the pros and cons and watch the market a bit too! Keep us posted!