To minimize disruption during the transition to Salesforce Billing, migrating historical payment methods ensures continuity in payment processes and customer convenience. Providing comprehensive training and enablement for end users and admins is crucial for smooth adoption and effective use of the new system. Comparing invoices from both systems helps verify accuracy and consistency, ensuring that customers receive correct billing information, thus maintaining trust and satisfaction. These steps are in line with Salesforce's best practices for system migration and adoption, emphasizing the importance of data integrity, user preparedness, and continuity in customer-facing processes during system transitions.

Salesforce Sales Cloud is the system of record for the sales process because it contains information about opportunities and opportunity products, which are used for forecasting and reporting. It also integrates with other clouds such as B2B Commerce and CPQ to support the cart-to-quote process.
https://help.salesforce.com/s/articleView?id=icx_b2c_crosscloudengagement_systems_of_record.htm&language=en_US&type=5 In the described sales process, Salesforce Sales Cloud serves as the system of record. This is because the overall pipeline, including opportunities and opportunity products, which are central to forecasting and sales management, is managed within Sales Cloud. While CPQ and B2B Commerce play crucial roles in quoting and online purchasing respectively, and the ERP system houses product and primary pricing information, it is Sales Cloud that integrates these elements to provide a comprehensive view of the sales pipeline and performance. This alignment with sales processes makes Sales Cloud the authoritative source for sales-related data and activities, aligning with Salesforce's best practices for sales management and forecasting.

the CPQ B2B Commerce Cloud Connector is an unmanaged package that allows B2B Commerce and CPQ customers to sync products, pricing, quote requests, and orders in both clouds.
https://help.salesforce.com/s/articleView?id=sf.icx_b2b_cart_to_quote_connector.htm&language=en_US&type=5

The two proposals a Solution Architect should recommend to track adoption of migrated functionality from Classic to Lightning are:
1. Track Adoption Rates within the Lightning Usage, and monitor a change in metrics within existing reports and dashboards. C. Provide the CIO a list of the User Stories around the new functionality and the Gap Analysis done between Classic and Lightning.
Tracking adoption rates and success metrics is one of the best practices for driving Lightning Experience adoption, as it allows UC to measure how well users are using the migrated functionality and identify areas of improvement or feedback. According to 2, monitoring a change in metrics within existing reports and dashboards can also help UC evaluate the impact of Lightning Experience on their business performance and user satisfaction.
Providing a list of user stories around the new functionality can help UC communicate the benefits and value of Lightning Experience to their users and stakeholders, as well as align their expectations and goals with the migration project. According to 3, providing a gap analysis done between Classic and Lightning can also help UC understand what features or customizations are available or not available in Lightning Experience, as well as plan for any necessary changes or enhancements.
https://trailhead.salesforce.com/content/learn/modules/lex_migration_rollout/lex_migration_rollout_enabling