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To change the database connection within an interceptor in NestJS using Sequelize-Typescript, you can do the following:

1. Define multiple Sequelize configurations in your app.module.ts file, each with a different database connection:

@Module({
  imports: [
    SequelizeModule.forRoot({
      ...
      config: {
        database: 'db1',
        ...
      },
    }),
    SequelizeModule.forRoot({
      ...
      config: {
        database: 'db2',
        ...
      },
    }),
  ],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

1. Inject the SequelizeService into your interceptor:

import { Injectable, NestInterceptor, ExecutionContext } from '@nestjs/common';
import { Observable } from 'rxjs';
import { SequelizeService } from '@nestjs/sequelize';

@Injectable()
export class MyInterceptor implements NestInterceptor {
  constructor(private sequelizeService: SequelizeService) {}

  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    // Change the Sequelize's default connection to another database
    this.sequelizeService.set('db2');

    // Execute the route handler
    return next.handle();
  }
}

1. Call the set() method of the SequelizeService to switch to the desired database connection.

Now, when your interceptor is executed, the Sequelize instance will use the db2 connection instead of the default db1 connection.