I think the mulch will be mostly mixed in as you remove the old plants and dig holes for the new ones. A new layer of additional mulch will give your garden a fresh look, and the plants will like it also.

My 2 cents.

Susan

Much depends on what you used as mulch. As a general rule, those mulches that break down fastest & that are smallest in particulate size tend to tie up N in the soil to a much greater degree when incorporated into the soil. An example would be hardwood chips. Other mulches, made from products richer in suberin (pine, fir, hemlock, or redwood bark, etc.), a lipid that makes it difficult for micro-organisms to cleave hydro-carbon chains & break the particles down, makes it a little easier to adjust your N applications because the rate of decomposition is much slower. No definitive answer in what I said, but you're on your way to the answer when the rest of the pieces of the puzzle are supplied.

Al

in texas, you might find that more than a few of those annuals are longer lived than you might think- especially if you got them from a franchise store, who tend to clump tender perennials in with the true annuals...and if you deadhead your annuals, most will rebloom for you much later into the year then they would for me up here in PA, where we lose the sun in september.

Al's right about what mulch you use mattering, but if your soil is less than ideal (I have one friend in texas whose soil I couldn't tell from beach sand, and one who's on a flood plain, with serious hardpan clay) then tilling organic matter- any organic matter- into it is a good thing.

you might also want to look into 'lasagna' beds (lots of references on this site, and some good links) which would allow you to balance the nitrogen out right in the bed, by adding different things, and composting it 'on site'

Chinacat indicated that some of the 'annuals' will live through the winter in your area, especially if they are well-mulched and in a protected area. I am in Amarillo; snapdragons, pansies, and dianthus will live through the winter on the south side of the house, and Columbines (a perennial) will grow new leaves in winter.

I put down a thick bark mulch in winter to protect my roses that are exposed to the north, and roses on the south side don't need it. I remove the bark in mid-March so seeds from annuals have a chance to germinate and grow. Sometimes I put a thin layer of bark to slow down weed germination and help retain water in the soil. To enrich the soil, I pull the bark away, add compost (or a cup of bone meal and alfalfa pellets for my roses), and put the bark back. Over several years, the bark will break down, and I can add more as I need it.

You might wait until Spring (late Feb.-early Mar. for you) to make a decision about removing mulch or spading it in to see what lived through the winter and to see what will come up from seeds. You can always thin seedlings and prune plants that lived. If your mulch is too thick, seeds will have a difficult time. Or you can always dig out the old and put in the new if that is what you want.

If you are concerned about what it will look like in the winter with dead, dry stems sticking up, just wait for things to die back and pull out or rake the dead stuff, leaving the live vegetation and mulch. Although most of the soil in the DFW area is very dark, rich loam that will grow about anything, there is a wide variety of soil types. You will need to decide what type you have and enrich the soil as needed.

Most of the time I cut off the old annuals at ground level, and plant the new ones next to them. I clear the immediate area of mulch, put in the annual then spread the mulch back. I do this by the hole, not the whole bed, but I guess you could rake off the mulch.

Soil will get mixed in in either case. But that's ok, it will help decompose the mulch. I do remove chunks of clay, as they don't look good on top of the mulch.

Mixed advice here. Having lived in Houston for 17 years, I've seen common impatiens live for several years (they actually get quite tall). However, Fort Worth gets hard freezes and snow storms that Houston normally doesn't get. There is a nice group of people on the Texas Forum that can give you advice better suited for your particular climate on this question.

Here is a link that might be useful: Texas Forum