Recommendation for membership plan structure

Hello support,
I hope you can provide some guidance. I am struggling with a plan structure for a website for a professional organization, similar to your "Society Club" example in the documentation. Here are some of the most important parameters:

-only registered members may purchase memberships
-there are multiple membership plans
-all plans begin/end on same date (July 1 to June 30)
-most plans have different prices
-once a membership is purchased for the year, member may not change membership categories until next membership cycle (should not be proposed other membership options until end of membership cycle)
-certain content is only accessible if user has valid paid membership

I have had the website working to my satisfaction for the last 9 months with memberships being updated from backend by business manager, but am now ready to allow error-proof front-end membership purchases. I have experimented with multiple configurations (exclusive vs. non-exclusive, free parent plan and paid child plans, propose/do not propose upgrades, etc) but either wind up with no memberships being proposed for those eligible or additional memberships being proposed for those who already have one. Thank you in advance for your response.

Joomla: 2.5.9
CBsubs: 3.0.0
CB:1.9

-only registered members may purchase memberships

This is already the case for CBSubs. A user can't purchase anything in CBSubs without being registered. If you mean you only want already existing registered users to see the plans then edit your plans and on the Workflows tab set "Allow registration:" to "No" or disable all plans on registration globally within CBSubs > Settings > Global and set "Show subscription plans at registration:" to "No".

there are multiple membership plans

You can create as many plans as you like within CBSubs > Plans.

all plans begin/end on same date (July 1 to June 30)

You'll want to use the "by calendar" durations on the Pricing tab of your plans.

most plans have different prices

Not an issue, each plan can have its own price as configured on the Pricing tab. A single plan can not have multiple prices beyond initial and regular price/duration though.

once a membership is purchased for the year, member may not change membership categories until next membership cycle (should not be proposed other membership options until end of membership cycle)

You can set conditionals for your plans on the Workflows tab of your plans for Upgrades so it will only allow upgrade when the other plans are expired.

certain content is only accessible if user has valid paid membership

This is always the case when it's protected by CBSubs. As you did not detail what Content is I'm not sure what more to advise.

Krileon,
Thank you for your prompt and specific response, definitely A+ customer service. You addressed all of my questions. Of particular help was the recommendation to set conditionals; this was the piece I was missing to control the correct presentation of subscription opportunities. In the plan workflow, I chose the static "this condition must NOT be met to allow upgrades", choose all of my plans in the "Any of the following plans subscriptions" and must be in state "active", my site is now allowing subscriptions to those without a current plan and is no longer proposing other plans to those who already have one. Thanks again.

Awesome, glad you got everything working to your satisfaction. Please don't hesitate to let us know if you should run into any further issues or have any other questions.