Just to clarify the whole Kickstarter thing (and this is ONLY for Kickstarter and NOT other crowd funding platforms)...
When you pledge any amount towards a Kickstarter campaign you must provide credit or debit card information at that time. However, no money actually changes hands (no charges are made) until the campaign completes its scheduled campaign time (usually 30 days, but can vary from campaign to campaign) AND is successfully funded (i.e. reaches it's goal).
Here is an example:
I launch a Kickstarter campaign seeking $100,000 dollars lasting 30 days. If my backers pledge $100,000 or more in 30 days than my campaign is successful. Everyone that pledged gets charged their pledge amount at the end of the campaign and I receive the funds (minus Kickstarter's fees) shortly thereafter. If my campaign does NOT reach $100,000 in 30 days, no one gets charged anything and I get nothing... Campaign is over. All or nothing... Again, this just applies to Kickstarter NOT other crowd funding platforms.
To the OP's question about changing your pledge during a campaign, backers are allowed to change their pledge as many times as they want or even cancel their entire pledge at any time during the campaign. Once the campaign is over, that's it... Your locked in. So, in this case, you could simply change your pledge from a pebble time to a pebble time steel.
Keep in mind tho, that some "tiers" or rewards are restricted to a certain number of backers (commonly referred to as "early birds"). If you give up an early bird spot and someone takes your place and they all fill up, your out of luck.
Sorry for the long post... Kickstarter addict here (if you couldn't tell

) .